After 20 years the tranny is probably gone
The cheap & nasty test is to put the mower on slope, turn the engine off and give it a little push.
If it rolls easy or starts rolling & does not stop then the transmission is shot.
As an example I use a tilting trailer to pick up mowers
A good tranny will need to be pulled off the trailer
A worn one will roll easily after a slight pull
A stuffed one will start to roll the instant the trailer tips.
Some times you can stretch the life of the tranny by draining it & refilling with heavier fully synthetic oil 20w50 as JD uses 10w50 or 10w40 .
But from now on that tranny is on borrowed time.
They are rebuildable if you can get your hands on a pump & motor .
