Are you good with a voltmeter....or a screwdriver?
I’d check if voltage is getting to the solenoid and starter.
Basically when you turn the key, and all safety interlock switches are satisfied (sear switches, PTO off, etc) key switch passes a small current to the magnetic coil part of the starter solenoid (the small wire).
When magnetized, it mechanically pulls closed a high current contactor switch (the big wire terminals), passing power to the starter. This is probably the click you’re hearing.
With a bit of research, there’s some ways to bypass starter contact with a screwdriver or jumper cables, etc.. and energize starter directly to test starter. Make sure tractor isn’t in gear or can move when attempting this and standing in front of tire.
Many times, there’s a simple fix like cleaning where the battery’s negative terminal connects to the tractor’s.
