You're using the gasoline engine to drive the impeller, right? All you're looking for is a way to steer the thing, since it already moves forward under its own power.
In that case, go after the problem: Steering.
Some ideas:
Mechanically brake one wheel or the other (is there a differential?)
Put a third wheel or track on to positively steer it
Drive one wheel electrically, and stop the other wheel when necessary (the electric one can run backwards...)
Ditch the auger and put a propane flamethrower on it.. (eh, scratch that one...)
I'm sure you can think of more.
It seems to me that making it track straight is an issue. I know that when I use my 7 HP blower, I have to wrestle with it to make it go where it's aimed - sometimes.
Oh, and in any case, put a fail-safe kill switch right up front, so if it's coming after someone they can whack the switch and kill the ignition or something. You never know.