Get a goped-sport engine or a bigfoot engine...There fast for what they are(22.cc) and you can mod them and also have clutche's.They dont use chain's,insted they use a thing called a spindle...One of these: The engines shouldnt cost much for a decent one...But go on this site for more info http://www.psukracing.com/index.php?cPath=126 I might aswell show you my bigfoot's engine: With a little modifaction's..Ive seen those engine's beat 110cc pit bike's and catch up with bmw's. And yeah they are only 22.cc and sound like a 2 stroke 125cc with a racing exhaust on. They also arnt no cheap crap either what break's down so easily.