As a metalworker, I'd say cut the frame, weld in a motor plate, get the plate down as low as possible, the lower the plate, the shorter the chainpath, and gyroscopic effect of motor minimized. A high mounted engine wants to ' torque ' the bike to one side during acceleration
Most 4 stroke guys do away with pedals, if you need them for legality, check out Graucho's photo collection . In my opinion, welding
[ brazing ] is needed to build a safe bike .