I can't think of many things that were different. However, early frames had 1" head tubes. In around 1992-3 the upmarket frames went to 1 1/8" steerers, while the cheaper ones were still 1". Otherwise they all took 28.6mm front mechs, 68mm BB shells etc.
There wasn't much front suspension in 1991, so you'll need a fork with 63mm travel...from a later year. Marzocchi Z3 or Pace would work well and are still possible to get bits. Many others especially elastomer forks can be problematic.
Ride it or milk it!