It sounds like you are on the right track. The ignition switch has 4 wires. Orange is the 12 volt crank wire sent to the starter via the pto and brake switch. The Red with the White tracer is your battery feed wire it should be on two terminals at the switch. It also should be protected by a 30 amp fuse. The Red with the black is your 12 volt key on function that should power most everything that is key power activated on the machine. What I would do if possible when it wont crank make sure the Red with the White is powered up in the crank position. If it is and the Orange is absent of power the switch is bad. Reason I say make sure the Red with the White stays powered as there is a splice were both of the Red with the White are tied together as they both feed the ignition switch. Sometimes a bad wire splice or terminal connection could loose power when a draw is applied. A lot of times you will get errant voltage enough to read on a meter or light a test light but disappear when loaded. One simple thing to check are the female spade connectors at the switch. Some times if multiple wires are in a molded plug they can be held rigid and not be pinching the male spade on the switch. Just maybe the other switch was replaced for that reason. Let me know if I can help you out further.