I want to buy a pocket bike, i need advice. PLEASE HELP!!!!!

i got one for 50$ that is junk basicly the engine is shot i think...and one for 80 where its kinda banged up and its a super bike(the big ones) but its got a new engine in it...i think its a ninja replica the kind with dual exause, lights,horn. well i was gonna put money into that but i dont think it will be good for racing... basicly i wanna buy one either a super bike or a pocket bike, although idk where to start. i would rather have a superbike becuase they are bigger but will i be able to get the same performance out of them as the tiny ones? can i get some links to some nice ones to start looking at...

also should i go 2 stroke or 4 stroke...im not gonna go to a track and race but id love to race around my neighborhood

http://www.minipocketrockets.com/rsr-mini-bike/

im looking at that one...
or this one mabye...

http://www.minipocketrockets.com/x7-...r-pocket-bike/

im also keeping this in the back of my head...
http://www.ioffer-autos.com/i/BRAVO-...SILVER-7732831

how are they all upgrade wise...

ok, id recomend a x12 4 stroke 4 speed semi auto, or an x7 4t 4speed semi auto.

tell me why

bump bump bump

i cant see the ones u put (the url goes back to the advertise? thing) but id say go with the x12 or the x7 (4 stroke) i know badazztoyz.com sells both teh x12 is a tad bit more expensive but u get what u pay for and i think it'd be a good buy plus u dont have to mix oil ;-)

would it be better to get the x15 like i plan on?

yes, i think so. x12 has mroe parts for it to buy, x15 doesnt.

i might keep it stock...which is better stock?