Thanks for the tips, Casey. I finally got all of the pieces separated from the old panel, and yesterday found I could order the black plastic panel itself, plus the wiring harness with plugs to fit the various units in the panel, plus a new tach, from Torrensen. They were extremely helpful in sorting out what was available (harness is coming from Georgia, via Michigan). The plastic piece was broken by the rust expanding around the body of the tach - that caused the thin corner that surrounds the tach to break into a separate piece that I could have glued back, but for $28 got a new one. The tach itself has actual rust holes through the body - suprising that it still seems to work fine, but certainly not a sealed unit any more. The tach body, and the hold downs for the tach and for the buzzer, are made of very rust prone cheap metals, while the rest of the parts seem to have weathered the water pretty well (ignition switch, buzzer, switches and lights). Some of the pins inside the wiring harness plugs are also well corroded, one broken off as I separated the halves of the plugs, but again, everythhing was still working until I opened it up, although clearly marginal as far as reliability in tough going.
For others with engine panels mounted in the cockpit, I would suggest a quick check of the seal where the panel mets the fiberglass. Mine was mounted to a teak panel that was then mounted to the fiberglass, and it leaked where the teak met the fiberglass.
In the end, this was a good chance to really clean up the main electrics in a 25 year old boat, without the need to suffer a breakdown at a bad time before discovering the need to do it.
1985 Marshall 22 catboat