this dude has a kit that doesn't need him to replace the back/front wheel with a motor wheel.
I kindda like that. the reason is I have decent wheels, and this method seems like the most convenient... and probably cheaper too since it's just a motor instead of motor + wheels like some of these kits are selling.
They are great look up crank motors most sites have kits. They attach to the chain and provide more power because you can still change and use your gears. The drawback is they can be noisey and and if you live in area with a lot of rain there is more maintenance required electricscooters.com has a couple of kits