I never bother working out how to keep motors cool. Let them get hot during the 3 minutes of combat and then cool after. Gearboxes, mario has some nice single units that he made a while back I believe from some steel plate. It's a more complex process making your own gearbox combo than just using a drill motor.

I learned this the hard way back with hornet 1. Decided to be smart and not use drill motors and use golds instead. Made some nice little wheel box units but those weighed more than a drill motor and i didn't notice a great deal of difference in the performance to be honest. Although I may crack them out at some point again.