I imagine that the engine RPM is fairly low (2000 - 3000 RPM), so why
not attatch a small steel vane (or more) on the pulley on the crank
shaft (or even the alternator pulley) and use an integrated Hall Effect/Magnet
assembly to sense when the vane passes through it. Use the PIC to decode
and display the RPM. These things work up to 130C so temp shouldn't
be a problem. The units are fully sealed and it shouldn't matter if the vane(s)
pick up a bit of sump residue, as long as it is not metallic (unlikely).

Some of these engines have a tacho drive which comes off the injector
pump. Maybe you could use this to drive a similar sensor setup and as
it is cable driven it would be remote from the engine compartment. I
made a digital speedo which sensed the rotation of the magnet
assembley inside an old speedo housing and it worked quite well.


Regards

Tony

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