After spending a little more time prowling that site I MAY know what your problem is: these toilets seem to be designed to use onboard pressurized flush water instead of sea water...which would mean they have no intake pump, but instead are teed into the cold water line in your fresh water system, using a solenoid valve wired to the flush button that acts as a "faucet,"--opening to allow flush water in, closing to stop it.
If you already knew that much, it's possible that the solenoid valve has failed...those don't have to be brand specific and are available from a number of sources. Or, one of your boat's POs may have wanted it to use sea water...in which case a remote
intake pump, some tubing and some changes to the wiring
would be needed. Check for that and to see if needs replacing (Jabsco washdown pump would work for that), or maybe just a new impeller.
That's the best I can do for you till I know more of the specifics of your particular toilet. Hope it helps.