This setup uses "off the shelf" parts without any modification.
The motor turns a 40T gear which has a 16T gear attached with 2 friction connectors. The pinion gear is attached to a drive shaft that turns a chain connected to the rear axle.
It doesn't matter which way the motor turns, the device still scoots along the floor rather well. My tests were done on carpeting. I know it's rather bulky, but I wanted to see if I could mess the gears up with a "reasonable" load. There doesn't appear to be any signs of wear.
The pictures below are from all sides. Click on any of them for the higher resolution shot.
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