I own a pest control company,,,,,,if you keep it in a barn, put the "candy bars" in tamper proof rat boxes around the entire interior of the barn,,,,if it's outside,,,I would leave it uncovered, and take the rodent bait ( cubes not pellets) which has a small hole In the center , put a 8-12" piece of bailing wire thru the hole and make a ring out of it with 3-6 pieces per ring, you can use that ring to attach it to area inside the tractor where the rodens are active,hang it from fuel lines, or wires,,,,the bait is formulated with grain seconds , so they are apt to take that as opposed to your wires and hoses, just remember to secure it it where it is not going to interfere with something that is moving
Personally I would fet one of those instant sheds or carports from TSC,,,,the above poster is correct in that the rodents don't like being sxpos to predators, so leaving it with the hood open in a well lit area will help. I deal with people classic cars all the time and if you don't use it at least every couple days then your at risk.
If you have a brn then an old milk room is the best rodent resistant area in a barn
Mothballs ( naphthalene ) really doesn't work unless you can keep it in an enclosed environment, it works in the someway that it works on humans , it irritates your nose,,mice are born and die within 15-30 ft of the someplace and survive by knowing their surroundings intimately,,,.
KILL THEM YOU'LL BE BETTER OFF',,,,,,,,TRUST ME
Joe