If you can, buy some 24V fans from Technobots and fit one for each motor, the bigger the better. That might help them last longer. You could also bend some Heatsinks out of sheet copper and attached them to the motors with thermal paste.

We have a single 40mmx40mm fan on Binkys 160HV for the weapon. It has never got hot even in heavy testing! A bit of air circulation can do wonders :-)