It's easy to power a bike with a 3-1/2 briggs. It's alot tougher if you want to keep the pedals. I understand in some states you need functional pedals to stay legal. Have you thought about front wheel drive? Or mounting the engine above the rear tire?
It begins, cut top tube and downtube. Going to bolt two small tubes to seatposts & downtube to lengthen frame , and sleeve top tube. Secure the sleeve and tubes w/ loctite on hardened bolts.