I would start at the beginning and verify the basics. Is the radiator fins and pipets clean. A good pressure washing should accomplish this. Has the engine cooling system had a good flush and cleaning? Get the garden hose out, open up the drain fittings (does the coolant draining out look clean and green?) ie: any oil or other crud. Flush out the cooling system. Refill with water for now.
Fire up the tractor and go to work. Does it still over heat? If no, drain the water and refill with proper mix of antifreeze. If it does over heat. Check the radiator with a pressure tester. Does the radiator hold pressure? If not find the leak (if not able to find suspect head gasket) With pressure tester still installed on radiator, fire up the engine and get the temp up. Does the pressure build beyond a reasonable amount (5-7 lbs.)? Does the pressure gauge spike and pulsate? If yes, very likely a head gasket.
If my memory serves me correctly (and it rarely does

) These engines can accumulate crud in the cooling system over time from failure to change the coolant and flush periodically.
Hopefully Mark777 will see this post and jump in here and straighten us all out. Mark is a wealth of information on these machines.
Where ya at Mark!

Which reminds me that I haven't seen him post much lately.