Welcome to the club. I have wasted dozens of hours search for a new rear master cylinder or rebuild kit for my 1984 ZN1100. They just don't exist.
I did have one very knowledgeable sales person who went above and beyond in trying to get me one. In the end I was told that in the 1980's Kawi was experimenting with a whole whack of different master cylinder designs, many of which where only produced for a year or two, so parts inventory was scarce and when they are gone, they are gone.
You might find one on Ebay and if you ever find a different one that fits, please post here.
This EBAY listing states that the caliper with slave cylinder is compatible with your bike. It's $65.98 new. I don't know if it actually is. I had to use a few search search words to find it. EBAY's own search engine did not list it.
I looked at ones similar to this but first I should note that my ZN1100 and the GPz550 use different rear master cylinders.
What I found, and what appears to be the case here, is that the Ebay link is for a "universal fit" Chinese master cylinder.
I cannot say if it will fit, but my concern is that they don't provide the critical bore diameter and given that their spec says (and I quote) "Fit 110cc 125cc 140cc 150cc PIT PRO PIT Bike Trail Bike Dirt Bike" I would suspect the bore is way too small to stop a 550 cc street bike. It appears that some Chinese vendors are willing to do whatever it takes to sell their product. Sure, with a hacksaw and a welder you could make it fit, but it won't stop the bike. Buyer Beware!
That's a big problem, sorting through the junk. I'm able to machine the heck out of anything to make it work, but you have to start with something you knows works. Rear calipers are so similar in design, I wouldn't think it would be hard to modify something to work. I don't have that bike, so there's nothing I can offer. Most bikes don't need much to stop the rear wheel, but something rated for such a small series of bikes probably wouldn't do the job. That's probably why it wasn't listed right off. Sorry.
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