As far as I remember the inner screws were rebound adjusters, with the outside (usually blue or black) being preload. I think on later models one side was compression and the other rebound. If its simply the top of the adjuster that is missing/broken then I doubt it would cause a problem further than not being able to adjust that side.
As for a spares, I doubt you will find one for that age fork new. WindWave (UK importer) were a nightmare to deal with for spares even at the time, never mind now! Your best bet would be wait for a set on eBay that are dead in another way and scavenge them for parts.