You might be able to mount a gear to one of the outdrives and run a belt from the front to the rear. I don't think this would be practical, but this was the first out of the box idea that came into my head.
The easiest option would be to replace the diffs/gearbox and come up with a different way to mount the motor. At this point you would have a hybrid design. However, you might be able to combine the best parts of the Kyosho suspension with a belt-drive drive train.
I'm currently racing VTA. Check here for rules/info: http://www.usvintagetransam.com/