1981 GPz550. what is the go-to replacement master?

need to replace a front master on my 1981 GPZ550. The orig is of course long gone out of stock.

Is there a proven replacement that will look / work like the old Tohiko?

No eBay APP ID and/or Cert ID defined in Kunena configurationDo you mean aftermarket? Perhaps, but why don't you go to Partzilla and see what other models had that MC and then hit eBay?
Here is one listed for a KZ550:

No eBay APP ID and/or Cert ID defined in Kunena configurationThe GPz550 use a 5/8" bore master. That ebay jobbie posted above is 14mm which is smaller.

The EX650 masters work nicely and can be found for cheap on ebay. I put one on my 750 and it's been great.

Stick with the OEM brake parts from Japan. I do not trust the master cylinders from China and you can forget finding levers and rebuild kits when you need them. I've been using late model EX650 and er-6n master cylinders and clutch controls on my projects for years. Ebay has hundreds of those parts, so good luck!

I don't care how bad it runs, the brakes have to work and stop your ride. There are a lot of aftermarket brake master cylinders, and some are very cheap. I'd stay clear of the cheap ones, but that's just me. Replacing with another if the same would be my first recommendation. If you wanted to change to one with a remote master cylinder like on the more modern bikes, or one from a modern bike, just about all of them bolt to our handlebars, and companies like Goodridge can customer make hydraulic lines for you. Just because it says "fits..." doesn't mean it is a direct factory replacement.