Oops. Bloggers' videos don't loop so it doesn't go 'round- Check out the web site...There's lots of neat stuff..
I found motor in wheel suppliers for bicycle conversions, and conveyor belts (drum motors) but I missed the obvious search words “wheel motor” until I ran into this interesting innovation from Michelin.

Anyway, I found a UK company, PML that produces a range of “wheel motors” sized for vehicle use. I have not had a chance to consider the various choices or to work them into a design, but they appear to be sufficiently small as to enable the many wheel, split carriage design I favor. (lots of traction, very tight turning radii, both up and down and side to side)
Designs will be forthcoming.