Skiing has changed drastically since the 1980s when the SX90, 91 and 92 were designed. Skis have also changed.
If you wish to ski the same skis as then using the same technique the old Salomons will probably work to the same extent they used to.
However ...
Lateral movement in boots is vastly more exagerated with skis since the mid 1990s and technique on groomed slopes and racing has changed completely. A boot that concentrated on forward flex patterns is not as good for todays equipment.
The Salomon rear entry SX series did not have a lot of control over foot fitting - by that I mean below the ankles and foreward (not expressed properly I know). A modern boot should fit the soles of your feet, forefott width, ankle width,e tc.. to the best extent possible. There just wasn't that much adjustability to the Salomon.
One final comment - with modern skiing techniques on groomed trails there is a lot of emphasis on angulation from ankes, knees and hip joints. The SX91 does not allow for a lot from the ankles and actually feels like a cast on your foot.
I skied Tecnica for years (TNT, TNT AVS, TNT Explosion 10, Icon) and since then some Head boots. None of the modern boots flex in the same manner as the old SX series Salomons - including Salomon.
Note that I ski an old pair of SX91 Equipe once per year on Retro Day with my old Atomic ARC 203cm SL skis. The skis are from 1993 and although obsolete are far less so than those boots. They are awful!
Go to a decent bootfitter and they should be able to recommend and find a boot that actually fits your foot and lower leg.
Mike