I can tell you if you use the search bar that will help. Someone made a thread i think the summer about it. I might be thinking of a bungee but I know there is one out there, just an FYI it costs like 400 bucks
How much experience do you have with mechanics and fabricating. If you know how to work on small engines you can get a motor dirt cheap off of craigslist, but expect some problems. If your not confident in your skills, you can buy a 212cc motor from harbor freight for less than $120 (look for predator). Also your experience and accesses to a welder can limit the size of the engine you can use due to strength concerns.