Tank steering is simply left/right forward/reverse.
You want the drive system to be electric correct? The only things you want powered by the gas motor are the blades, blower, and some kind of generator system? If so I would be checking the local ads for electric wheelchairs. They have adequate power to move the mower around and up hills, have tank steering, move at a reasonable speed, and you wont have to worry about gears and belts, etc.
I dont know too much about the generator thing. The only thing close in RC terms is a system used on large scale planes and I don't think it will work for 12v batteries. A car alternator might be easiest?
The actual radio system that you will use to control the mower needs at least 3 channels. A channel can control one thing, like a speed controller or a servo. You need one channel for right side drive, one channel for left side drive, and one emergency engine cut-off. Additional things like lower/raise deck, deck clutch, or chute rotation all need their own channel. The radio you use must operate on 27 Mhz, 75 Mhz, or 2.4 Ghz. These are ground frequencies. I would stay away from 27 Mhz because basic Wal-Mart toys use this frequency, and I would also stay away from AM because it is prone to interference. This might all sounds like gibberish but you will understand once you start deciding which radio to use.
hands without shadows cause my hands move so fast my shadow can't keep up