Welcome badpenny to the forum, glad you joined us
A vintage bike would make a good motorized bicycle as long as it is not all rusted up to where it's full of holes. But even then it can be repaired. A lot of members look for old bikes to motorized.
As far as the motor what are you looking for? 2-stroke, 4-stroke, center frame mount, rear mount? Friction drive, chain drive, belt drive?
I like the 2-stroke center mount...I think they look cool on a bike.