[jimfortytwo]

You should have one last chance to aquire 12V motors as a drop in relaplacement when you arrive at your regional, so long as FIRST doesn't run out (which is very possible). Read the updates closely for the details.

In response to your original querry over applied voltages and heat: It sounds like you've done some reading, and have heard people say over and over again to keep fisher price motors away from stall, and run them at high rpm so the cooling fan can do some good. When you half the voltage to 6 volts, obviously the cooling fan slows down, but much more importantly you have reduced the power (heat/time) that needs to be dissipated by 1/4. So running at lower pwm values should prolong the life of your 6V motors.