Scale Tank rebuild, what motors to use?

Brian, I'm going to keep it on sticks,(Spectum DX7) simply because I need 6-7 channels for the turret / main gun / big bang stuff. At the moment the drive is controlled by both sticks Two speedos across two channels. I think this is the best way, as its easier to control turning at low speeds, although I have seen a plug in mixer that will mix left/right/forward /reverse on one stick (like the Tamiya's)
The gearing at the moment is via a big transfer box with quite a few gears in it. I was hoping bin this and use a chain drive system,( to save a bit of weight) if I can get a motor/controller that turns slow/smoothly enough.
How should I work out the gearing for this one? do I measure the tracks or the drive sprocket?