Good, now I can look into the diagrams. Your tractor is a 1989 model.
OK, for the engine to roll over, the following has to happen. (A) The Fuse (20A) has to be good. (B) The Ignition Switch has to function correctly, (C) The PTO Switch has to be in the OFF position, and (D) The Brake Pedal has to be depressed.
This of course, has to take into consideration that (A) It is wired correctly, (B) The wire and connectors are in intact and in good quality condition, and (C) that devices (ie: Switches) are in good condition and function correctly.
Sounds like a tall order but if you have a Test Meter (VOM or DVM) it can can traced down within a few minutes.