From what I've looked at so far, it seems the friction drive setup similar to Wane's In frame Predator fricton drive build
would fit my means and abilities now.
It still seems easier to me to mount the engine above the rear wheel. Actually, many frames wouldn't allow an in-frame mounting, and I think I have such a frame.
I think I would like a centrifugal clutch on a pulley to belt to a pulley on a friction drive (2 shafts + the wheel).
Then, if the engine fails, the wheel has to turn the friction drive, belt, and pulley outside of clutch. The clutch, being disengaged, prevents the motor from turning. That's my estimate of how things would work.
Come to think of it, I've never up-close seen a motorized bike. But that shouldn't be a problem.