8) log onto "CRAIGLIST". i found a complete kit w/battery pack for $175. good price, but i didn't need it. because of such low demand here for any type of motorized bikes, it's a buyer's market for resale engine/motor kits.
also, it depends if you're looking for 15mph/10-mile range, or 25mph/50-mile range.
speed cost money.
BTW, i built a 25mph/50-mile cruiser. zero vibration, almost as quiet as a regular bike. i gave up on it because the bike weighed 135 pounds, batteries need charging daily, and i live in a 2-story walkup.
i also installed a 35cc engine friction drive kit on my folding bike. now my bike doesn't fold, but it gets 100+mpg on gas. at 40 pounds, i can easily carry it up to my apartment. however, i was totally surprised at the engine vibration and noise, after riding my electric bike. i have less than 10 miles on the gas engine, but i think my electric cruiser was faster and accelerated quicker.
Myron