Re: 26" La Jolla
It has been done with a China girl, but it was a difficult install. Oversized downtube requires an oversized front engine mount. You must buy an intake manifold that offsets the carb to the side of the seat tube or make your own. Some aluminum frames crack after being motorized.
If you really like that La Jolla, I would consider a rear mounted friction drive setup, IMHO.
“You're only young once, but you can be immature as long as you like" -Unknown