21 years is pretty far past the expected service life of a suspension fork, so this could be tough to find something off the shelf.
I see Universal Cycles has a SID rebuild kit for ‘99-‘08. You may just need to buy it and figure out what still fits. Or figure out ahead of time what changed from ‘98 to ‘99.
Most of what these kits contain are O-rings, which you can probably get elsewhere if you can do some careful measuring of the old ones with digital calipers. As far as the wiper/seals go, you may be able to find those separately through someplace like Enduro Seals or SKS. Might need to contact them and talk to someone.
I would recommend checking the slider bushings for any wear/play. If they are worn, I would not bother putting money or much time into the fork.