I've had this bike for almost 4 years and love it. But if I had it to do over and could have known I would want a motor, I would have bought this:
Nirve.com - Large Image
It is about the same price and already has good brakes. With the low-swooping rear frame, (where the rear tire goes), you wouldn't even need a chain roller/tensioner. You would probably have to make a special engine mount and modify the pipe though because of the super forward pedals. I could live with those mods I think.
Anybody ever motorized one of these?