Eh, I don't do fwd on anything. Mid drive sounds promising, I've seen motorcycle motors fitted to them. I was thinking of a scooter drivetrain retrofitted, but the frame alone ourweighs a standard 50cc scooter frame minus drivetrain...the back wheel probably outweighs a scooter backwheel.
A mid drive, and maybe a front freewheel kit. This is just one part of the plan and a wheel hub motor of highest wattage for it's 50-65lbs(as is) self and my 250lbs self will be wonderful. It might be wide, but it's still a 20in.