Its your boat, do what you want. Biggest concern would be will the new digital dash be waterproof or at least water resistant? Gauges in boats are in a hostile environment, where even normal auto gauges will fail. RPM should be standard Kettering type ignition, assuming I/O Merc 140. Oil pressure is prob 10-80 ohms, temp likely 20-120. Trim is adjustable and somewhat objective, adjusted at the drive. I set mine(zero'd) by putting drive all the way in, then adjust the cup to read down. I scratch built a custom engine analyzer as a thesis project a long time ago. I calibrated oil pressure using a lab pressure gauge and master cylinder, the temperature sensor using boiling water. RPM calibrated with square wave generator, etc. Honestly it sounds like a great project, but in the end, I spend my time boating watching and enjoying the scenery and my company, not concentrating on dash(unless its a new engine of cours). Given that, insure you have at least two Idiot lights, one for the alternator, which also shows a loss of drive belt causing loss of coolant flow. The second idiot light is oil pressure, VDO makes a great sensor with both oil pressure gauge and idiot light outputs, so best of both worlds. Lastly, speedo can be accomplished with gps interface, but a pressure sensor on a pitot tube could work ok if you could research the pressure vrs speed chart. GPS is far more reliable, pitot's get kicked out of position, clogged by debris, and occasionally issues with tubing arise. Take pics, would love to see your progress.
