I know what your taling about, I bought a x19 from usa-pocket-bikes it came 3 weeks later. I took it out of the box and some of the fairings were cracked from overtightening the screws, and they almost never answer the phone. Then I had to go through a long procedure to get new fairings. After that they came 2 months later.
There are tons of performance parts available for these 4 strokes. Some offer a great improvement, and some don't. Choosing your parts wisely will make the difference between money well spent and money wasted.
Iridium Spark Plug
Iridium plugs are a fairly inexpensive upgrade. They do not need to be gapped like other plugs. It is very unlikely that you will notice any increase in power. You may however have a bike that's easier to start, and more resistant to fouling. If you aren't on a tight budget, it might be worth the money.
Igntition Coil
Aftermarket ignition coils are intended to generate a stronger spark. Most of these engines will never need a stronger spark. You generally need a stronger spark when you drasticly increase cylinder pressure, from things such as higher compression, turbocharging, supercharging, etc... Under increased pressure, the spark can be blown out. Given the small gap of the spark plugs, unless your stock coil has seen better days, you should be fine until you build that high compression, cammed, nitrous, 114.
CDI
Aftermarket CDIs are advertised by some companies like they will turn your stock bike into some ground-pounding monster. I have never found this to be true. There are different styles of CDIs available. Some aftermarket CDIs only offer a change in the rev limit, others offer a timing advance, while others change the rev limit and the timing curve. I have installed CDIs that advance the timing with both positive and negative results. In one case, the bike acted as if the timing was too far advanced, and ran poorly. In other cases, the bikes seemed the same as stock, and with others there was a slight improvement. From my experience, I would save my money, at least until after some other mods.
Inner Rotor
Sellers claim the inner rotor kits will make a 4 stroke rev like a 2 stroke, because of the reduced rotating mass. I personally have no experience with one of these kits. I have been advised to save my money however. I was told they will rev a little higher, but they really aren't worth the price.
Exhaust Mods/Systems
Modifying or replacing the exhaust helps a little bit. You may or may not be able to feel the effects of exhuast modification and aftermarket exhausts on a stock engine. You can usually see more of a gain on modified big bore engines. One thing you can certainly do is hear the difference. If you like to hear your engine, and you aren't in an area where neighbors will object, exhaust mods are nice. You can do the mod in the article posted below for next to no money. You can also buy complete systems or buy a dirtbike muffler and weld it in place of the stock silencer/muffler.
Carburetor
I see a lot of people swapping carbs on stock bikes. Personally, I have never felt enough difference to make it worth the cost of the carburetor. Carb swaps can make a big difference on big bore engines, they also help on cammed engines a bit. If you really want to put a carb on your stock 110, 20mm carbs are a good choice. I have ran a 24mm carburetor on a 110, but I had to jet it down, and still didn't see any real improvement. 22mm carbs will usually work well on modified 110s. My suggestion would be to wait until you get a big bore kit or at least a cam, to upgrade your carburetor.
Intake
There are a few different intakes availabe for this engine. They make short, medium, and long runner intakes. There are aslo intakes that typically come with carbs to match up to them. If you really want the best bang for your buck, wait on an intake until you buy a carb, and then get an intake to match the size of the carb. You can also polish your stock intake, if you have the proper tools. I have always used a Dremel tool. To get all the way insdie the intake, you will need the flex shaft attachment. All you are trying to do while polishing the intake is smooth out the casting flaws. One other thing you can do, is assure that the intake matches up to the carb, heat spacer, head port, and gaskets. This will assure that the air/fuel mixture has a smooth clear path into the head.
Head
Aftermarket heads are available for these engines. You can also port and polish your stock head. The race heads are much better than porting and polishing your stock head. The race heads offer larger valves, and improved ports. Porting and polishing your stock head will make a very slight difference, and if done wrong, can cause a loss of power. I would opt to buy a race head after or along with a big bore kit. It can make a difference on a stock engine, however it will do even more on a big bore setup with a cam.
If you choose to port and polish your stock head, I suggest you pull the head off of the engine. It can be done with the head intact, but you run the risk of metal shavings and dust causing damage to your engine. It is much easier to get the head clean with it off of the engine.
Once you have the head off, you are mainly just cleaning up the ports. You can slightly enlarge them, but I suggest only those of you that understand engines well do any of this. While you have the head off, make sure your intake port matches up to your intake. Before you re-assemble the engine, clean everything very well with carburetor cleaner or brake parts cleaner to assure no metal shavings or dust is left.
Head Breather Kit
Head breather kits are advertised to release pressure in the head, which will allegedly allow the engine to rev faster and make more horsepower. One problem with this claim, is that the crankcase pressure is already vented, rendering this kit a waste of your cash.
Big Bore Kit
Big bore kits are an excellent choice. They can transform your mild 107cc (referred to as 110cc) engine into a high compression 114cc. It does this using a domed 54mm piston, and over-bored 54mm cylinder. This kit, by far, made the biggest difference of any mod that I have done. I would highly recommend a big bore kit to anyone looking for more power. It does require removal of the head, sleeve, and piston, so it is important to have some mechanical ability. On some engines with the bottom mounted electric starter, clearancing some fins may be necessary. When combined with a race head, cam, and 24-26mm carb, you can basicly double the power of a stock 110.
Camshaft
Camshafts (cams) can change intake and exhaust valve lift, duration, and timing. These changes can allow more air and fuel in and out of the combustion chamber, change or extend the powerband, and in some cases, alter compression. These changes can allow your engine to rev higher, and create more power. There are two common sizes for camshafts in these engines. Be sure to measure your cam before ordering one. You should find it to be either 68mm or 73.6mm.
High Volume Oil Pump
A high volume oil pumps move more oil than standard oil pumps. This helps keep the upper end lubricated. If you have an oil cooler, it can also push oil through it more effectively. Installation of an oil pump requires drilling the oil passage as well as a lot of disassembly.
Oil Cooler
Oil coolers are designed to keep your engine temperatures lower, by cooling the oil. Cooler engine temperatures can aid in extending the life of your engine, as well as keeping it from losing horsepower due to heat. Oil coolers will work with stock engines, but I have found them much more effective after the addition of a high volume oil pump.
Engine Swap
Engine swaps are another option for getting more power. There are many 120cc and 125cc engines that will bolt in. If you are looking for more power, but don't wish to pull apart the engine, this may be a good choice for you. You can often find packages including a larger carb, cdi, clutch cable, and anything else you will need to swap engines. Of course, once you swap engines, you can add performance parts to it as well. Some people have built the 120 engines to exceed 160cc.
If this engine group cannot satisfy your needs or wants, you can also choose a different style of engine. This is very likely to require lots of welding and fabrication skills, and is not for the inexperienced.
I hope you have found this information useful. Use any infromation here at your own risk. I can't be held responsible if you choose a bike you don't like, or if you mess something up trying to modify it. This is merely a guide. Thanks for reading!
Re: x18 mods? 
