Kind of a hard call...
Is this still a working tractor?
Are there any problems, like fouling plugs?
Is the oil milky, watery froth at the breather tube or under the oil cap?
If no, it might be happier leaving it run without a thermostat.
But if you think bringing the temp up will help, and it will need to have some running time up to temperature to have any benefits, as in some extended time working under a load, then try it.
Just keep an eye on the temperature and coolant level. Sometimes if the radiator is clogged they will want to overheat with a thermostat in. Won't know till you try.