Starters should not "keep going on" you. Yes, they do go bad, but not one after another. That indicates some other problem underlying this issue.
Do you have adequate gauge wire going to the starter solenoid? Undersized wire would cause low voltage and that could prevent the solenoid from fully engaging, damaging the gears. If the starter is misaligned with the flywheel it could cause binding and wear out a starter more quickly. Undersized wire from the battery to the starter motor could cause low voltage and cause the motor to heat up.
Jinma, who makes the Nortrac tractors, has come out with a new design starter with compound gearing that is more efficient and more powerful. You might want to see if one of the new ones will work as a replacement for the original. Those who have used them think they're great.
I'd suggest you contact Tommy at Affordable Tractor Sales for advice and to see if the new-style starter will work for you.
Rich