Everybody wants into the bike game! (& they always want to slap engines onto the cheapest bikes they can possibly find... )
Good luck, & don't get anybody hurt on your bikes! (Knock on wood)
Problem is you almost have to in order to get any business, because most people will want a moped for anything over $300. Yea it's not the best idea, but think about what you would do with $500 and no mechanical aptitude or knowledge about these things.
Best thing to do is try to conceal the bikes in some sort of theme, so people will see something that resembles an antique conversation piece rather than a motor someone slapped on a bike. You'll end up with a better profit margin per bike to the point where you can replace the cheap-o mounting hardware, Walmart white-walls, and etc.
Of course, this only works when you've got a pretty decent metal shop at your disposal.
Check out the vendors that sell theme'd bikes & you'll get a better idea about what I'm talking about (start with venice's site).
I remember a few years back someone used to sell cheap-o bikes to kids for ~$300 at the skate park. I'm pretty sure he was getting rid of at least 2 or 3 a week, but these were broke 14-15 year old kids.