A little of the old oil will have leaked out via the Piston rod, and will have thickened up being 20 years old.
If you could change and or top up the oil, you might find leakage is pretty small if it's for occasional use, you might need to to it up yearly.
Worth trying before throwing it away.
Hopefully that little screw undoes and allows access to the oil chamber?
The classic fix on disposable motorcycle shocks was to drill a hole into the oil chamber, pump out the old oil, get some fresh oil in, and seal the hole with a self tapper
.
(Personally I'd tap a thread
)
If you find it's coming apart, with any luck the piston seal is a standard 'o' ring, available from a hardware store.
Worth a try before replacement. You might find a replacement shock challenging because travel has increased so much since then