Most of the older Nauticats I have seen have had the Blake LAVAC head systems which use the Mark V Henderson diaphragm pumps. A very much superior system to the Par, in my opinion. Reworking the Henderson pump is quite simple, especially when using a rebuild kit, including diaphragm and valves.
You can remove the top half of the pump without dismounting the whole assembly from the bulkhead. Just put all the screws and parts in a bucket with disinfectant for a while, then clean them one at a time, carefully chipping the deposits off with a screwdriver. If not replacing the rubber parts, break away the stuff as best you can, without chipping or puncturing anything.
Going back together, lightly Vaseline all the mating surfaces so they slip into place nicely.
The four screws that hold the discharge valve are tricky to seat, so do this part carefully, to save having to repeat the entire reassembly.
Remake all the hose connections with silicone sealant and if the clamps are rusty, put on new ones and line them up carefully. Test with clean water and stop any leaks.
Nigel Calder has a good write-up on this in his book, Boat Owners Mechanical and Electrical Manual.