Disconnect the existing wire to the fuel solenoid, at the solenoid. Run a new wire from the run position of the key switch to the fuel solenoid.
Run another wire from the start position of the key switch to the starter solenoid.
Should start and run and also will shutdown when you turn off the...
What year is your tractor? I would be careful that you really have oil pressure. Also directly powering the run solenoid will bypass all safety shutdown functions
19 when using the key roll back position. 50 is used to apply power to the glow plugs when the key is in the start position. Why? Maybe a pollution control thing
Probably the safety controller. Look for a black/red wire coming from the START terminal of the ignition switch and going to pin 17 of the controller. After the tractor is running you could clip this wire and see if the problem goes away. You can always reconnect the wire when done with this...