I've got a Kings 80cc motor that works nicely on my touring bike. The problem was, I bought it for my restored Columbia Cruiser which the webpage said it would fit, and it would have. Only problem is that it would have neccesatated drilling holes through the frame, which there was no chance in hell I was going to do. So, I needed a bigger frame and I chose my touring bike. Unless your rock hopper is pretty big, there is a chance you might need to drill some holes in the frame.

I don't want to discourage anybody from building up bikes like this, because its a good and cheap source of experience for those who have don't know much about motors. Don't get me wrong, I have a lot of fun with my 80cc touring bike, but these things aren't fit for daily commuting. A frequent problem you will experience is the chain pulley getting misaligned, and other jamming and stuff is a common occurance, at least in my experience. Also, bicycle tires are not meant for the continuous speeds these things can put out. If you wanted a very cheap daily commuter, a mini bike would be good. If your just looking to have fun, motor kits are great.