Moisture accumulation in the transmission is a common, ongoing problem in those tractors. The best solution is to keep it inside, or at least under a cover. Even then expect to get some condensation.
The oil will need to be watched and changed when it begins to look milky. Might try pulling the drain plugs occasionally, best done when it's been sitting unused for awhile, see if you get water out before it gets emulsified.
Be sure the shifter boots and the boot around the top link are good.