I would like to use a 66cc Sky Hawk with a shift kit. I would love to hear the disadvantages of the shift kit if any. The hard part is going to be deciding what bike to use. 1 option is a 21speed towine. I love the ride but mounting the motor has some issues. I may try to extend the front derailer hanger and see if that will fix the biggest bump I see on this bulid. The grip shifters will also need to be dealt with. 2nd option is a Cosco bought Motiv. Its a fair bike for the $200 I paid for it a year or two back. It is called a full suspension bike but the rear "shock" is more of a dampener then a real shock. I can see very little if any movement in the rubber shock. Should be OK. Front motor mount will need some help due to the odd shaped front tube. It has grip shifters too. 3 is my Gary Fisher 29er. It's a hardtail and just a great bike. Not sure if I have enough room for the shift kit though. Well fellows, what do you think? I would love to do all 3 and may one day. But for now which looks like it would be a good first build? Thanks for your input.