Cruddy master cylinder

G'day, I just pulled apart the cruddiest master cylinder I have ever seen, it looks like it has never had the fluid changed since 1977. It is a KZ 650. I am in the process of deciding whether I can resurrect it but I feel that the cost of a repair kit won't be enough as it looks like the cylinder bore needs a stainless steel insert or maybe I could get away with a bore.
When I pulled it apart I was wondering what the role of the diaphragm was. It must allow the brake fluid to pass thru it, I thought maybe it won't allow moisture to pass thru it?
Has anyone fitted a second hand master cylinder or an after market one and did it operate properly? regards Neil

Is

Yeah that's the one but it didn't look schmick like that one, gee it was in a horrible state I should have taken pictures of it

This is off a kz1000 A2A but it is the same principle

4TheKZ1000, These look great I have sent you a PM regarding these Master cylinders regards Neil