Scooter guys use freestyle footpegs for drives with their weed-eater motor conversions. They mount it to the motor shaft and shove it against the tire.

Google/eBay Warrior EV, I think there is/was a friction drive with a knurled drive wheel and I just saw one for auction. Friction drive seems quick/easy but no good in rain and tire wear will increase greatly.

Personally, I'd suggest a front 26" hub kit. About as easy - remember you still gotta wire-up controls, batteries and stuff with either design.