Are you set on ignition fixes? Weld a loop to a rear rim bolt on each side (on the inside). If you run a chain with lock between them the tractor will only move until that chain hits the transmission or tries to go behind the fenders. They would get 3 feet or so. I couldn't imagine that someone would have brought the brawn to drag it without turning wheels. If they did your ignition lock won't help.
Heck, what about turning the wheel all the way to the right and then chaining a steering wheel spoke to a radius rod? Adding anything to the ignition system is just giving you another opportunity for a breakdown at some point.