Typical "chain of command" on those tractors is the start signal begins with key switch. One tiny wire from there leads to the PTO switch. From there another tiny wire to the neutral switch. Then it goes back up to the actual start relay which is under the cluster in the steering column area.
You might look at the fuse link. That's on the right side of the motor near the intake manifold and grid heater (if you have one) I've seen a few failures there where the fuse wire isn't quite up to task and will actually melt the insulation and the wire yet still perform but only partially carry the load.