We had two lovely Wilcox-Crittenden Imperial 51 MSD's on Red Ranger. We took out one to install a composting head back in 2012 (see "Painful Head-ectomy.")
We've kept the other working pretty well considering it's something like 37 years old at this point. It's a heavy piece of machined bronze. Really elegant (in some ways.)
And by "we", I mean CA. My role is limited to handing her tools while she fiddles with the darn thing.
The imperial model has a foot-operated valve for the flush water. The valve has a spring-loaded return that — of late — doesn't return anymore.
We can't seem to get the set screw tight enough on the shaft that turns the cam that opens the flush value to allow water to flow down the inside of the bowl. I think it's reached a point where we need to carefully file the shaft flat around the set screw to get it to hold.
Instead of fiddling with this, CA votes for a new MSD. One that's not 37 years old. One where parts and rebuild kits are readily available. (We have a rebuild kit from the previous owner, complete with gooey packing for the pump body, and some rubber valves of unknown age.)
The Raritan PH Superflush seems to be really, really similar to the old WC (Water Closet or Wilcox Crittenden, both are correct.)
We're aware of the possibility of a shit-cano when we disassemble the old head, and I think I have a strategy for dealing with this. (Back in 2012, the shit-cano was a bad surprise.) My solution involves placing the head over a wide, shallow dishpan before disconnecting the hose up to the anti-siphon loop.
5′ (60″) of 1½″ hose is 106 in³, about 0.45 gallons, 3.7 pints of water. At this point, it's mostly fresh water, but, it helps to be prepared.
We're waiting for delivery to our nearby West Marine, where we can schlep it to the boat.