The few that I recall of the Nonsuch boats had Volvo (if direct drive) and Vetus (if sail drives) diesels -- it's been a long time. The one I recall had a small Volvo diesel.

Cars, as I recall act in "reverse". I believe, BUT AM NOT SURE, that boat diesels operate in lock-step -- i.e., as heat increases the resistance, the gauge temp goes up. The pressure switch would seem to be more straight forward. I would suspect that it could be a European (German) VDO gauge set. I'm not sure you can use a conventional replacement, if that's the case. VDO should be available -- if not expensive.

I guess I'd check the heat with a infra-red thermometer FIRST. If he doesn't see a very hot block, and no steam behind, then I'd proceed to check the power to the gauges. The neutral would be the first thing to check at the gauge panel by wiring it directly to a ground and seeing if the voltage changes.; then I'd check the voltage of the "hot" at the gauges; and at the engine. Them "cold" and at no pressure -- then with the engine warming up and the throttle advancing.