Something that might help.
Make sure the wire going to the starter is at least 4 gauge, I would use 2 ga.
Add an additional wire, the same ga used as the power feed wire, from the battery negative terminal to one of the starter mounting bolts.
This added wire I'll give the starter a clean path for the return current., straight to the battery.
A quick check would be to take at least a 4 ga jumper cable and connect it from the starter mounting bolt (get to bare metal) to the negative post of the battery. Start the tractor and see if it helps, if it does then do a permanent job.