I would suggest a north west shortline gear box on the larger wheel axle...under drive and the motor in the floor with a flywheel. Being that the front truck wheels are different sizes you'll probably never be able to drive them both the same speed.
An amusing totally old school idea...and this is waaaaay left field, is the old athern HO scale rubber band drive system. You'd need a drum nearly as large as the large drive wheel and a front to back running shaft to drive it off of- a slightly twisted band connecting them together.