Hello. I think putting a hub motor on a shopping cart would be the best bet. You’d have to have both wheels in the back be direct drive, so you can have reverse. You may be able to wire a couple of joysticks as a throttle/reverse. One for each wheel. The vehicle will be much like a tank. That’d be a fun project, actually.
Really though, what would be the intended use of the bike? For commuting, off-road, hauling a bounty from Costco, or riding kids around from thing to thing? You say you ride a lot, would you be interested in converting your existing bike?