Here are tips to try and cool it down.
1. Cut cooling holes in your body.
2. Put a heatsink on the motor.
3. Run it with the fan on the motor.
4. Gear it down. Chances are it's geared WAY to high. The lower you gear it, the slower it goes. But your buggy will run cooler.
5. Check the driveline and axle carriers for anything that could cause binding.