The easiest thing is to get a direct replacement from IH. You can pick up an ignition switch from your local autoparts store.
First thing to do lable each wire going to the switch. At the same time make a sketch of the function (make a wirng diagram) of the original switch. Post #1 goes to ??, perhaps battery. Post #2 goes to ??, off. Post #3 goes to ??, ignition run. You need to know where each wire goes and how the switch is supposed to function.
Visit the parts store and ask to see their catalog on ignition switches. Compare your switch function/wiring diagram with the wiring diagrams in the catalog. Once you select a switch, copy down the wiring diagram from the catalog.
