The safety switches, if bad, should prevent the starter from engaging, so probably not that.
Check the other plug wires for spark. 1 & 4 share the same coil and 2 & 3 share the other one. If 4 sparks, then you probably have a bad wire on 1. if 4 doesn't spark, but 2 & 3 do, you probably have a bad coil. If none of them spark, you probably have a bad ignition box.
Of course, all this assumes you have good grounds on everything and no shorted or open wires in the ignition circuitry.