I agree on the motor. I have one of these, that my daughter learned to ride on. It has a 2.5 hp 79 cc Greyhound motor on it. It pull's me fine at 200 lb....and once up to speed clips right along in the upper 20's I'm sure.
I have a 6.5 predator on my Boonie Bike...which is a heavier frame, compared to yours, and it dumped me real quick.
Please help your Dad have a good experience for his first time back on his bike, and source a smaller hp motor, whether vintage or new, and you'll be glad you did....:thumbsup:
BTW...Thanks for doing this for your Dad !