Too bad this ends so soon. Answered
If I had another month, I was planning on making a nice instructable on how to make a good motorized bike, doing 150mpg, with 7 speeds, and a max speed of 80mph.. Although I don't think anybody will get up to that fast. You'll still be able to petal, and it's street legal (considered pedal assist) No welds required. The modification process can be dumbed down to a sheet of metal, hacksaw, and a drill. However, I'd recommend a sheer for cutting, but that's just me. The total cost of the project would be a bike, plus $200 in parts, plus $200 for the engine. Maybe I'll submit it to the green one instead, 150mpg is pretty good. That is if I get all the parts and have the time to put it together, plus do a write up.