That old WC head is worth preserving if you can.
I doubt that you will be able to get a 'service kit' that is purposely made for it though.
I've installed a few service kits and I'd guess that if you can get a service kit for the oldest WC head they have that you might be able to use many of the parts. Some parts will not work because of the foot pump but I'd say it is worth a shot.
Mostly those kits consist of:
- a joker valve (could likely be made to work with your old unit)
- a few rubber parts that serve as valves or bushings
- a few springs or odd shaped metal parts.
You could probably make your own service kit if you knew what shape/size parts you need with the exception of the joker valve which is purpose made and looks a bit like the mouthpiece of a Clarinet but made of rubber.
Another brilliant idea I had is to post the pics of your old WC head over at Peggy Hall's forum (she is the "Head Mistress" and knows her heads): The Head Mistress - SailboatOwners.com
She wrote a book about heads and boat odors. The book is pretty good too, I have it. It might help you understand some of the ways each part is supposed to function. I'll bet you could figure some of this out for yourself if you took it apart, piece by piece too.
Beautiful old head, worth saving, if you can.