If the stop solenoid is removed and the starter spins over good, there is nothing electrical keeping it from running except the lift pump under the tractor providing fuel to the injector pump.
These things are simple, mechanical injection, no ignition, no ECM, nothing. If you have fuel to injector pump and compression it needs nothing else. If your stop solenoid sticks engaged (or you remove it) you can't shut the thing off electrically.....removing air or fuel is the only way to stop that little motor, well you can stall it with enough load.