Have you checked the fuses behind the gauge panel?
If you have, remove the plug from the fuel shutoff solenoid and unscrew it. If it starts it's a bad solenoid. As a heads up, you have to reinstall the solenoid to shut it off.
On the right side of the tractor you'll see the injector pump. The fuel shutoff solenoid screws into the left side of the pump. It's cylindrical and has a wire / spade connector plugged into the end of it. You can gently use a pair of channel locks to get it loose. If it doesn't start with it removed, plug the spade connector back in before reinstalling the solenoid and turn the key on and off to check operation. The plunger should move in and out.
I should mention that the solenoid can be hard to see without a flashlight even during the daytime.