I'm hew here and have yet to build a motorized bike but have an interest because a friend of mine wants one and I suggested to him the engine to use. I want to build a motorized bike with a 50cc 2 stroke or 80cc. I want to build the bike from a fixed gear road bike with a flip flop hub using the free coast side to pedal and the fixed side to run the engine drive chain. I figure the cog that is on the hub right now is too small. It is maybe like a 17-19 tooth and it should be about 29-40 something to not put too much strain on the engine right? My question is, is there a company that I can buy a cog from with an appropriate number of teeth to run an engine that would also screw on to the fixed gear hub?