20 years old would mean a 91 which would put it right on the edge of suspension corrected geometry (they started tweaking the steering head height and angles to compensate for the increased length of suspension forks around 91-93, depending on the manufacturer). If you give me the model name I can look it up in our database. An older 63mm fork with a 396mm a-c length like a Rock Shox Indy, Mag 20/21, or Manitou 2 won't adversely affect your handling too much. For what your bike's worth, I'd look for something used on kijiji or a deal on old NOS stock at a mom and pop bike shop and get someone to swap it out for you.
There are 3 different steerer tube diameters and 2 different stem systems. You need the right one. If I had to hazard a guess without a model name I'd say 1 1/8" threaded.