Just to confirm what's already been said... Like Andrew pointed out, the cog spacing varies on SunTour AccuShift indexed setups, so it's not a straight swap for other 7-speed freewheels. Not if you want it to index well, at least. Not too long ago, I had a 7-speed AccuShift freewheel (on a '91 MTB like yours) that was just plain worn out and after searching in vain for a replacement, I wound up replacing the whole drivetrain with recent Shimano parts.