You have a few choices, First find a wiring diagram, I have found them online at the toro website, usually in the owners manual section. And get yourself a decent Multi purpose electrical meter, I bought mine at sears for less than $20. The purpose of a kill switch is to interupt spark to the spark plug or the power to the starter, some people perfer to do both. So Find the ignition wire off the key switch and put your switch there chose a switch that will allow you to ground wire, for the starter a simple single pole switch between theswitch and the solenoid. I like some of the suggestions of putting an inline fuse holder nad just removing the fuse. No switch to draw attentionn and the fuse holder doesn't look out of place. If you repair that mower you can figure this out. Check with the dealer, ask nice be polite and he will probably give you the diagram you need.