they did the same thing to my D-14 AC one year.
another year to my tiller, wood splitter and lawn mower engines.
I just have to keep packs of mice bait out in the shop for them to eat and hopefully go off somewhere else and die.
one year they built a nest in the area of our car, you could see the nest down through the vent between hood and windshield.
also a few good cats will do the trick.
at least you found it before you started using the tractor and started a fire