I used a 23cc weedwacker engine mounted onto the back of my crapper bike to push the back wheel.It runs great! Yeaterday morning at 2:00 AM, I finished making the latest mounts and driveshaft, and drove it down Bellevue Way and all the way back, without pedaling!
I have this great design going on now though, totally indestructable, kind of opposite of my old mounts that bent up after 10 minutes. It involves a cargo rack and a chain, but I can't explain it unless I have pics. I'll try to get 'em!