When I was in high school, everyone was doing this - though few were ever finished and none of them worked. I built a scooter at home with a hacksaw, a drill, some angle iron and old bike parts, so you should have no problem with all the tools in your shop. The transmission or drivetrain is the hardest part to work out, you need a low gear ratio between the engine and the back wheel, lower than you could ever get with bike sprockets. You can have a wheel such as the outside of a centrifugal clutch rubbing against the tire, or have two sets of sprockets with an intermediate shaft, like a lot of mopeds. Or you could look for a very large sprocket or pulley wheel and mount it on the back wheel. Another idea would be to use the front sprocket and pedal shaft that are already on the bicycle as an intermediate shaft, have the engine drive the big sprocket, and weld a small sprocket beside the big one, and have that drive a big sprocket on the rear wheel, of course the sprocket on the rear wheel is small, so you have to weld a big one on. Before you start welding stuff you can figure out the gear ratio (size of engine gear / size of wheel gear) by calculations and figure out the speed per engine r.p.m.: RPM x gear ratio x wheel circumference. that gives you speed in inches per minute, which you can convert to miles per hour. If you're using a chainsaw engine, aim for a top speed of 20 miles per hour, if it's too much higher then it just won't have the torque to get going. Your next biggest problem is the cops. I learned the hard way. Drive sensibly, don't run stop signs, and you stand a better chance of them ignoring you.