i dont like roadbikes or tenspeeds with thin tires,i road a tenspeed and ran over a small pebble and got a flat tire,what you think will happen with a motor on it and run over a pot hole?.you dont have to have a mountain bike,cruiser bikes have nice smooth tires and can take a bump here and there. with the friction drive you dont have to mess with chains,but no peddal start and you have to lift a bracket when stopped plus ive heard you have to replace tires more and i dont like that,maybe someone should make easy to install and under $200 friction kits with centrifugal clutches,but then sometimes the pull start might break. now with the chain drives you have to deal with a chain,some chain adjusting but once youve got it right its good for a while and has great power with no slipping.plus they have kits available and cheap. i'd go with chain or belt drive.
The bike to build with would be a large 26" steel bike with large 26" cruiser smooth tires ,springy seat, shocks front and back with rear swing arm,and rubber-steel engine mounts like a motorcycle. when was the last time youve seen a motorcycle with thin tires?