Normally when one part in the head needs replacing there are others that are close to needing replacing. I would buy the standard Jabsco refit kit and replace all the parts that comes in the kit. It is very simple.
If you are up to it you can always take the thing apart and rinse all the rubber valves etc. in vinegar and then rub them with a bit of cooking oil before installing them again (a bit of a shitty job).
To maintain the head in working condition as long as possible add vinegar directly in the bowel and pump it slowly through the system over an hours time a few times a year. Adding a few spoons of cooking oil directly in the bowl regularly and then pumping it through also works miracles on the Jabscos in terms of durability and smoothness in the pumping action. As soon as I hear the smallest squeak I give it a bit of cooking oil it makes a mega difference - I promise.