Ok, based on your private messages to me, you would rather not cut and weld, so the 1200w motor is out.. *I don't blame you - it was quite a job, and if it were not for a good friend of mine to do the brunt of the fabrication(with his spare scrap metal, MIG welder, and grinders/cutters) I don't think I would have bothered..
With this decided, you are going to want to go with the 750w/36v motor, and pump 48v to it - for max power..
Unless you want the yellow mx650, save yourself $100, or more, and buy the MX500, because you are going to replace the motor in either one, anyway..
Chris at TNC Scooters is great for all the parts you will need(~about $170 total in upgrades):
~$20 Extra 12v/12ah battery.
~$40 100a/48v controller
~$38 48v battery charger
~$65 750w/36v motor(*Swaps right in w/no modifications)
*Ok, How fast can you go with this setup? My Son(65lbs) saw 31mph max, using a 15 tooth front sprocket(not stock)/80 tooth rear(stock)=5.33:1 ratio. When we tried any larger front sprockets(to lower the gear ratio) it wouldn't go any faster. Probably because we exceeded the capability of the motor(Horse Power). *I weigh 175lbs and I maxed out between 26-28mph, so you would probably see ~28mph if you weigh ~135lbs...
*Now here is the deal. You have to decide how you are going to use the bike. If you want top speed, you will have to ride on flat hard surfaces all the time, because if you run a 80t/15t=5.33:1 ratio on dirt, jumps or hills, it will feel sluggish, and you may end up frying the motor.
*I tried the top speed thing for fun, but if you want to go in the dirt/jumps small hills(like my son rides) I would suggest a higher ratio.
Before we upgraded to the 1200w/48v motor, we had the 750w/36v in his bike. We tried a 8t front sprocket(80t/8t=10:1) and the thing popped wheelies, had killer acceleration, and had top speed as fast as a stock mx500/36v, 17mph - but believe me, it was way better than a super slow lame stock 17mph mx500 !!! We ride on this dirt course(see my vids), and my son had such acceleration, he left his buddies stock mx500's in the dust!
*Now, the stock gear setup is 80t/11t. This is a fairly good combination for the best of both. It went ~24mph top speed, and had better acceleration than stock mx500's.. *But, if you are going to ride on hilly/dirt courses where you never get near the 24mph top speed, you will have to monitor you 750w motor.. It can overheat.. *Remember, you are pumping 48v 50-100a to a 36w motor.. (*But, we never saw the motor get hot, no matter how my son rode it - when we had 9t or 8t front gears...)
*Now, lets get to my photos. They are of my son's MX1200. He rides this just like a 70-90cc dirt bike. Jumps it like heck, spins dirt out of the corners, ect... It's allot of fun, and you can pretty much get the same out of a 750w upgrade. *About the only difference is, you need to pay attention to the 750w motor. It will be getting the same power as my son's, but it is smaller, and can get warm/hot. If you take care of it, it will be fine.
*When you buy the MX Dirt Rocket, and the upgrade parts, - let aproldan and I know(by posting here) and we will help you through the upgrade process. When you are ready to start, I can send photo’s of where I put what(Extra battery,48v controller, ect) and step you through the process.
I have a mx65o, the yellow paint lol, and i am going to modify it using your steps. Is there anything you can tell me about how to get stared? My email is [email protected]
and I would love if you gave me help.