To start out, there are a couple different manufactures of these motors, there are some differences in the parts, so you need to determine what type of motor you have. Do you have any idea? The cylinder/head/piston is commonly referred to as the "top end" everything below it is the "bottom end". You can buy a complete bottom end or individual parts, depending on how much work you want to do. The biggest problem here would be determining if your top end will work with the new bottom end you buy? (what brand motor do you have?) So, pending your identification of your motor, it may or may not be easy to get replacement parts and have everything fit together properly. If, for instance, you have a Grubee motor, GasBike has 66cc bottom ends for $74, 48cc for $64. Bikeberry seems to have parts for at least two different brands of motor, there are all the sponsors linked on the sides of this forum that sell the parts for various motors, and then there are the sellers on ebay. (If you look at the PARTS section at BikeBerry.com they show an exploded parts view for all the various sections of the motor which is very helpful in understanding what your dealing with) So far I'm not exactly sure what "parts" you need if you go the repair route, if your willing to do the work to crack the bottom end and just need bearings someone will help you find replacements and that would be pretty cheap. Pictures are always helpfull.... Sorry I don't have a simple answer for you. Pass on any more info about exactly what motor you have and someone will help you find the right parts and give you the best advice, which may depend upon your skill level and/or desire to dig into some mechanical work.