If keeping it stock is your main goal, quite a few other KZ and GPZ front masters are identical or near-as-makes-no-difference to. Kawasaki liked to change things up year to year, so getting 100% identical might be a challenge, but there are many out there that are close- I had to switch mine, and the only difference is that the sight glass is a smaller diameter than stock for a 1980. But, according to my service manual it seems the 1979 was fitted with the small glass, so... who knows?
Also, depending on how corroded your original is, it may be worth the cost of a rebuild kit to try to refirb it. I was worried about mine, but turns out most of the pitting was in between the seals and not actually on any mating surfaces. A dowel with some emery cloth in a drill might clean it up enough to work, and the master seals have more give than people give them credit for.
There's also plenty of aftermarket MCs out there if you just want braking power without the need for "period correctness", but I can't really speak for any of those. Good luck!
