I have a spare used one kicking around somewhere, if I run across it I will tear it apart to see what all goes into it. There have to be modern parts available that will fit, I imagine tricky to find though
The extra tricky part is that our OEM unit seems to have the proportioning / splitter valve built into it - most other Mazda chassis of this era had external prop valves. Otherwise it may be possible to switch to another MC.
I recall having seen some 323 guys switch to a Mazda 929 MC, and even a Mitsubishi EVO master cylinder. My memory of these things is growing foggy. However after a quick look on Rockauto it seems the 323 uses a drastically different brake booster setup so this is probably not a viable option without switching the booster out.
I have the booster & master cylinder out of an early Mazda MX-3, complete with external prop valve, that I was going to try to fit to the GTXR2. A friend parted his MX3 out and I grabbed it, it appears to have the correct bolt pattern to mount to the firewall but I don't think I ever tried to fit it. I believe it had a larger bore MC which is what I was after to balance my brakes after doing the Galant big brake set up front