Welcome to the world of British Brake Boosters. You have three choices.
Shop for a new or rebuilt booster. Some fabrication may be necessary to mount it.
Send yours to someone like Tiger Tom to have it rebuilt or, if you know what you are doing, you can buy a rebuild kit and do it yourself but it isn’t as easy as it sounds.
Bypass the booster and use foot power. Once you get used to it, it is not too bad.
You will probably get lots of responses on this one but they will all address the same three options.