93 Rockshox Quadra top cap

I was helping my brother in law take apart his '93 Quadra fork and I lost one of the top caps. Now it's either replace it or find a replacement 1" threaded fork. Anyone know where I can find a top cap, and if a cap from any other fork would work? My other option is to have him measure the other cap, get the diameter and thread pitch to see if I can replace it with a metric bolt or pipe cap (he took the bike back to his place). The cap has a pre-set adjustment, but I don't think he'll mind loosing that ability.
Thanks