Temperature gauges work on resistance. I forget which way it goes, but in most cases, the greater resistance means a higher temperature. Something that people often do is put teflon tape on the sending unit when installing it. Unfortunately, the sending unit is grounded through it's direct connection to the engine block. Teflon tape is an insulator. See the problem? LOL Pull the sending unit out and make sure that it and the part that it is screwing into is clean. Then put it back in with no sealer etc. That's the typical problem that I would see with my many years of working in the automotive aftermarket.
If that's not the problem and both the gauge and the sending unit have been replaced, we need to start checking resistance along the wires. It should be pretty close to 0. If there's still an issue, make sure that the gauge is grounded correctly.
Gauges like this are actually pretty simple once you know how they work. Time to pull out the VOM (volt/ohm meter) and spend some quality time with the tractor out in the garage.