It's not the motors that you need to be concerned about overheating, but the circuit board. When you get it open check and see what board the tank has. If it's an RX-13 with the solid aluminum "heat sink" you definately want the extra cooling. The RX-14 boards are better and some say that a fan isn't even needed, but if you are going to upgrade to metal tracks, wheels, etc. I'd error on the side of safety and put one in. I have a tiger with metal gears and tracks and am happy with the speed. The extra weight slows it down and almost totally eliminates the "super spin" and they look better.

Here's a link with good pics for the basic mods. Check this forums FAQ section for a step by step break in procedure.

Enjoy!

http://www.rcgroups.com/forums/showthread.php?t=743482