I motorized my mountain bike.. I think I'll go with a cruiser with a more upright seating position next just for comfort. My lower back (i have back problems anyway) has been hurting lately due to the hunched over MTB riding position.
I have a cruiser with a gebe rear rack on it. Love the whole thing, but if I had to do it over again I would do it with a full suspension MTB. But I like sitting upright so I would also put the cruiser handlebars on it. Changing the handlebars would then mean longer cables. But the trade off is great. Much softer ride. The springers help, but there's nothing like that suspension on the rear. It's just that the full suspension bikes are all kind of short in the length, I like that stretch look and feeling.
My first bike was a Kulana Moondog. While it is an affordable and spiffy frame to use, I had a difficult time trying to fit the engine on that frame.
I wish I had started out with some old yard sale steel V frame mountain bike. In hind sight, that kind of frame would have made my first build go A LOT smoother. A frame with a little suspension already built in. I don't have the kind of money needed to retrofit a cruiser frame with suspension at the moment.
My first bike was a Huffy Mountain bike. The engine barely fits and I cannot position the motor in such a way as to square both the front and rear mounts and so I have been shearing off mounting studs.
I should have gone with the cruiser styles. I have had great success with the Kulana Moondog and the Schwinn Point Beach bikes.
I bought the wally world cruiser/staton friction drive as a 'proof of concept' anyway. I fully planned on upgrading after I got some miles on the current rig, and got to know more what I needed (and wanted) in a long-term rig.
I know that a shaft drive is a bit less efficient than a chain, but, when combined with an internal shifter hub, it's just about maintenance free, and, with a motor, the peddling efficiency isn't nearly as important as it is when you've ONLY got peddles available.
And, since I'll probably be moving to a wetter climate in the future, I'm looking at a change from the friction drive to a chain drive, also.