When your ignition switch starts to fail in the starter position, you can just wire in a momentary on pushbutton switch in parallel with the key switch to power the starter solenoid. Like the switch Ron pictured. Here is one looking like it belongs. I still use the key first, because occasionally it works and I have to turn it at least to the run position anyway.
It even fits the decor.
Relays are nice if you have voltage drop issues, or want to control current with lighter duty switches and wires. However, I had a standard relay fail on me at Sebring this year. It was the ordinary multi-prong 25 or 30A relay readily available at auto parts stores (not the round Porsche one), and one of the coil wires broke off where it was soldered to the prong which stuck up through the plastic casing. Given the finer than a hair gauge of these enameled coil wires, it is hard for me to imagine how it worked up the inertia to fatigue and fail. But it did. i suppose it didn't have quite enough slack between the solder joint and the coil, and perhaps the coil wiggled enough. A failed relay coil was not my first thought, so finding this took a bit of time. I think, for a race car anyway, that it may be useful to minimize the use of relays where you can.
Walt