While there may be merit in trying to create a replacement piece just because it's interesting, for something like this that needs to work reliably and stay "square", I'd just buy a replacement square of good quality and move on. Using parts from other sources is certainly a possibility, but I suspect there is risk with an exact match and I say that from experience. I have both a Starrett combination square and a smaller Starrett "regular" square. A few years ago, I bought new scales for them as I wanted/needed metric scales. They were purchased from Lee Valley and are excellent. The rule for the combination was an absolutely perfect fit. The one for the smaller square works fine, but is harder to keep locked because there is some slight variability in the casting...it's an older square I bought pre-owned many years ago. The combo was also bought pre-owned, but was in like new condition and a newer vintage. So sourcing parts across brands "could" present minor (or major) fit differences "just because" there's no standard here.