There are two rebuild kits available for the manual head. One is the rebuild of the pump, and the other is a replacement pump assembly. The latter is more money, but all you need to do is undo three hose clamps, disconnect three hoses, remove the pump assembly, and then install the new pump assemby.
The less expensive option is to do the same thing, but also completely disassemble the pump and rebuild it.
Hint: Make sure to shut the intake seacock before you begin working. Pump LOTS of fresh water through the system before removing the discharge line. When you disassemble the discharge line, the only thing that will backflow is the freshwater. Keep a rag handy and plug the discharge line to minimize backflow.