If your tractor is in otherwise pristine condition, and you got it at a reasonable price, you ought to treat it to a new cylinder.
Check sources other than Ebay. I stupidly assumed JD offered absolutely no parts for my 318 being 22 years old. I paid big EBay money for used stuff I could have gotten from Deere for less.
Either here or at Weekendfreedommachines.com, you could probably find someone who has done this rebuild. Even a hobbiest might want to be paid for his time (and equipment), so between parts and a reasonable payment for labor, you might be better off with a new one.
But back to your 1991 318 with red tranny fluid and multiple leaks. I don't like it at all. A friend has an 1800 plus hour '85 318 that has never leaked. Mine doesn't leak, my neighbor has never had a leak.
I think you really need to consider purging the system and switching to Hygard fluid. I don't know if there is a JD approved seal conditioner/stop leak additive, but that would be worth investigating.