Get RPM from vibration
Do both ends of the crankshaft disappear into inaccessible things? Is there no way whatsoever to get to any rotating part - the pump impeller, injection pump, cam belt/chain/gear, starter gear...?

Writing off any sort of variable reluctance or hall sensor on any rotating part (which seems far fetched, but for the sake of argument...) there are also other potentially easier ways to sense RPM than microphone - although automotive knock sensors are tuned microphones and you could get lucky with one of those. 

You could sense pulses from fuel injectors, exhaust manifold, injection pump. I'm assuming you have no alternator or rotating electrics at all, otherwise the ripple/AC from any of those is an easy win.

The LM2917 circuit is a very old one, and I don't really see why it's useful today - converting RPM to a voltage doesn't seem like a useful thing to do; If you are displaying it on a standard RPM gauge, then you want to be capturing the dominant/target frequency and filtering/conditioning it to suit the RPM gauge, which will expect pulses. If you are capturing the data with a microprocessor for transmission or logging etc. then the micro can count pulses just as happily as it can read an ADC, but the error will be much lower if you're just filtering the pulse train rather than doing a relatively vague f-to-v conversion.