Welcome to the forum.
There are many options using current technology that are superior in many ways to that 25 year old solution.
Most of the boat / monitor interface is VERY simple, things like DC voltage measurement and switch open / closed.
As an example, for the bilge pump, is 12VDC provided for the motor to run, or not.
Another example, for engine oil pressure, the engine has a oil pressure transducer, wired to the helm gauge, to provide an analog running real-time oil pressure display. The engine also has a oil pressure switch that closes at low pressure, usually about 5 psi.
The old original OEM system has two boxes, one that interfaces to all the stuff on the boat, another that is a display, connected to the interface.
The replacement will be very similar, just using current technology.
The wiring diagram provided w/ my boat was insufficient to fully understand the System Monitor, so I made my own supplemental wiring diagram page.
