RE: mechanically R/C'ing a joy stick
Strip the variable pots out of the joystick or buy ones with a similar resistance and operate those from the servos.
Good primer on 2 wheel drive steering here [link=http://www.4qd.co.uk/robot/start.html#sas]CLICKY[/link]
I was involved in a big fighting bot and built a couple of Antweights [ 150 g max ] from scratch lots of fun!