Soak the whole thing in brake fluid; it's part solvent, and might loosen it. Brake system cleaner may also work, but it evaporates quickly, unless you put it in the bottom of a container you can seal.

 

put a fitting in and pump it up with grease - that should hydraulically force the piston out part way at least.

I'd go to Kawasaki's site to see what all models used that master cylinder and buy a used one to replace it. You go through all the pages till you get to your parts schematics, pick the part number then click on the question mark in the pop up window to see what all models used it.

I'd find it hard to believe you couldn't buy a replacement really cheap from some on-line source like the Rice Paddy or Pinwall Cycle.

 

So I went to the link and found the part number, but when I click on it, the item status is "cancelled." Does that mean they don't sell them anymore? If I have to, I'll order an offbrand replacement, but I'd much rather fix this one to keep things original. I soaked the MC in brake fluid, fluid now runs past the end of the piston and out the brake line hole, but that darned piston is still stuck. I don't understand what it's stuck on though. The spring behind it is full compressed. The cup seal inside shouldn't be able to hold that much tension. I'm lost on ideas. Also, as a side note, the caliper piston is also stuck beyond compressed air. Have to fix the master to get the caliper piston out. This bike sat for 10 years with no love

 

I found a few used ones on eBay motors with a quick search; obviously, no guarantees there. I've had pretty good luck with CB450 parts, though; that bike is the reason I have an account there in the first place.