Disconnect the (+) battery cable before (if) you try this. If you ignore this warning, the tractor could come to life if you mistakenly close the cutout relay contacts. The tractor could motor forward under the power of the SG motor and damage property or endanger you or bystanders.
Open the regulator case and check the contacts on both relays inside the case.
When not running, the contacts of the CUTOUT relay should be OPEN.
The contacts of the voltage / current relay will be NORMALLY CLOSED. The issue might be that they are welded closed and therefore cannot open the ground to the field coils in the SG motor. If the relay contacts fail to open and the field coil stays fully grounded, no voltage regulation
OR OVER CURRENT PROTECTION for the SG motor is available.
You may be able to gently pry the contacts open with nothing more than light fingernail pressure peeling the contacts apart. Don't overdo the pressure, they should separate easily. If they separate with a "snap", you will most likely see evidence of corrosion or pitting. A points file or very light grit wet/ dry paper (400 / 800 grit) may get the regulator back in operation. It may only be temporary, but it would allow you not to guess when it comes time to open your wallet and take out some hard earned cash !!
Chuck