here are two ideas that should be checked first:

1. make sure the blade engage lever is fully retracted. If this is operated by a cable, the cable may be dragging and not allowing the mechanism to return fully to the off position. In this situation, a safety switch is wired in series with the cranking circuit and the circuit is still open.

2. When the key is turned to the crank position and the brake pedal is depressed, electrical power should be routed to the starter relay - through the smaller wire that is attached to a spade connection on the relay. Use a test light to see if power is delivered to this connection. The second spade on the starter relay is a ground connection. Sometimes there is no continuity on the ground wire connection to the tractor frame. This connection is frequently located behind the right rear tire.

Thanks bluemower.

1)The blade engage lever is fully retracted and is not operated by a cable. It has always been in the off position and the safety switch for the blades has never been pressed in any of my trials.

2) I will have to give this a try this weekend and report back. I've seen in a few forums the term solenoid used, just so I understand, are the terms starter relay and solenoid describing the same component?

I replaced the solenoid and still have the same symptoms. When I turn the key, nothing at all happens. If the ground underneath the wheel well lost continuity, how do I correct it?

Have you considered checking the fuse?

@bill_kapaun I already replaced the fuse as noted in the OP

Bad ignition switch?

Nope, replaced the ignition switch already as stated in the OP

Thanks sstracker1. I didn't do a continuity check on the relay/ground, I just checked to make sure everything was connected.

The starter being fried would cause there to not be a click or any kind of sound or effect when turning the key? Is it common for the starter to go dead if it sits for a few years? Is there a test with a multimeter that I can do to confirm its the starter?

A proper Ground Potential is Imperitive for all Electricial Components. As Bluemower advised all Safety Switchs (Interlocks) must be verified functional . As for the Starter yes when you hear nothing , normally you have a dead battery or starter . A clicking sound more likely relay or bad connection . The worst thing for a starter is to crank it for more than 30 seconds continuously (over heating) The second worst thing is to let it sit idle for 24+ months (brushs tend to stick). To answer your question yes you could try a test , directly spin test the unit on your work bench with a 12 volt power source . You can then better determine the bearing conditions without any load or interlock issues. If the starter was turning over a multimeter (Dynamic) ampere test would indicate if it was faulty due to excessive amp draw (load) due to windings breaking down or perhaps bad internal bearings. Static testing in place most likely would only confirm a Dead Short Scenerio ! P.S. Having additional external elect grounds never hurts , I do this commonly on numerous recreational vehicles , were excessive vibration can cause less than optimium ground potential and Electricial Gremlins !

You may have checked the fuse, but why isn't the carb solenoid clicking when you turn the key ON? You stated you hear NOTHING.

Fuse HOLDER may be bad?

Another Continuity Test , Sounds reasonable if not audible to me !