Easy trial and error performance HL mods.

The motors and gearing that came in your tank from HL is the probably the best solution.

But, the stock Tamiya 380 tank (not rc buggies) motors with stock pinions will drop into the HL gear boxes with no mods (HL gear boxes are a copy of an out dated Tamiya Design). That will make your tank a little to fast, but fun. The motors with pinion gears are cheap and can be found on ebay.

Try the site below for various low torque 400 motors and pinion selections. The tank will become scale speed slow, controllable and crawl up almost anything in the backyard or battlefield using low torque motors. I did not like this mod. To slow and not much fun.

http://www.hobby-lobby.com/speed400.htm
http://www.hobby-lobby.com/motoraccy.htm
http://www.hobby-lobby.com/pinions.htm