If you're still at this, I'd suggest removing the added solenoid and associated wiring and getting the tractor back to original configuration.
Once it's back to the way it was built, see if it work and diagnose it, and repair it the right way.
I work on a fair amount of old (tube type) electronics , and I approach things the same way. Removing the "improvements" that some Dreaded Previous Owner installed often simplifies the repair process and sometimes removes the fault altogether.
Keep us posted!
