I have a motorized bicycle kit-- which bike should I put it on?
In my opinion, unless the mountain bike has some kind of suspension system, it should depend on the kind of tires you're using. If the beach cruiser is using a high volume, low pressure style tire, I would put it on the beach cruiser. This would make for a way smoother ride than on the mountain bike, and give you way better traction, which is a big big plus for higher speeds. A treaded mountain bike tire would actually give you less traction, because there is less surface area of the tire touchng the ground. This is all assuming that you aren't going to be taking it off road, by the way. Hope this helps!
Sell the motorized bike kit and exercise yourself by pedalling the bike !
