Re: First motor bicycle
Since you do have the large 1 1/2" down tube, some of the vendors make a really nice mount that bolts to the front mount holes and has two holes for a U clamp. It works pretty well and you dont have to drill a hole in your frame.
Like Finfan says, it is very important that you get the rear sprocket runing as true as possible. This can be done very easy by ordering a machined rear sprocket adaptor and sprocket from Jim at Manic Mechanic (Creative Eng.) You will need no tools for that application as he sends the proper hex wrench's. I wouldn't build another bike without one.