My advice is to take what you have and improve it. With some small skill and/or a lathe or a good drill press you can put bearings in just about, not all but almost all, gears in a Tamiya transmission. The same can be done for anyone else' transmissions. Friction is your enemy, bearings are your friend, not only in the transmissions but in your road wheels and idler wheels. Every little bit helps. For the other manufactures, ditch the plastic or zinc gears and get either high quality brass or real steel gears with either bushings or bearings. For any bearings you need, use either Boca Bearings or Avid Bearings and get the rubber sealed units. Since all our transmissions are made in countries that use metric specifications just go to the charts in Boca or Avid and get the bearings in metric, not only for your transmissions but for your road wheels and idlers.

As for after market motors, you will have to know the rpm parameters you need for your application. Motors with flux rings will generally give you more torque but cut the speed a little. If you are running a light tank and torque is not that important but speed is, you can remove the flux ring and get an additional 5% to 10% rpm increase with the corresponding speed increase. The problem is we use direct DC motors, generally 7.2 volt. These motors are getting not easy to find with the advent of brushless motors and while some aftermarket electronic systems for our tanks will support brushless, Tamiya and most others won't.