Try these steps
I don't have a wiring diagram, but here are some generic steps you could try (not specific to this tractor). (In general, check that all the connections are in tact and not corroded etc.)
1/ Carefully bridge the positive power source of the battery to the small terminal (usually numbered 50) on the starter motor. Makesure the tractor is in neutral and keep clear of moving parts because that will cause the starter motor to start and turn the motor.
This passes the key switch and any solenoids and safety switches.
If starter motor starts turning goto step 2.
If not, check your starter motor (and the solenoid in the starter motor).
2/ check if there is a safety switch and see if you can bypass it.
Try starting with the key. If it starts, then safety switch is either faulty or the connections to it are not good or both.
If it does not start, goto step 3.
3/ check the starter relay.
On Bosch relays, I believe positive power is applied to terminal 85 and ground is terminal 86.
That should click when the relay is excited.
I'm not sure what relay is on your tractor, hence not sure if the safety switch plays a role, hence why in step 2 I talk about bypassing it.
If the relay works, then you should get positive power from terminal 87 (provided terminal 30 has positive power connected to it). When positive power to terminal 85 is stopped, terminal 87 should have no power coming out of it.
If the relay is not working, replace it.
If it works, then the key switch will need to be checked.
Let me know how it goes and we can go from there.
Once, you get it all going, remember to re-activate safety switches etc.
Apologies if the response is not clear - I have a tooth ache.