G3X RPM display

No, a continuity test is just testing for a low resistance connection. Diodes are semiconductors and to test them you need to inject a suitable test current and measure the voltage across the diode. That is what the diode test function on a multimeter does. I would be surprised if your diodes are faulty unless they have been accidentally connected to a low impedance voltage source like the battery or generator output. Have you checked your tacho sensor and your wiring?

Depending on what equipment you have access to, you could build yourself a tacho generator to test the circuit and the G3X input circuit. Unless you have a suitable oscillator module, you might be able to use a 9V battery and some mechanism of creating pulses. You might be able to use a microswitch that is rapidly switched by an eccentric 'lobe' in the chuck of a drill. This should be enough to give you an RPM readout on the G3X.

you can use the AC voltage range on a multimeter to see if you are successfully generating a signal.

Al, if you have a meter with diode test function then you can test the 1N4002 rectifier diode without the complication of my method.   Your meter will do exactly exactly the same thing internally.  However, you won't be able to test the reverse breakdown of the Zener diode.  99% of multimeters don't use a test voltage high enough to check a 7.5V Zener.

For the 1N4002 rectifier diode, put the red test lead on the anode (no stripe) and the black test lead on the cathode (stripe).  With the meter in diode test mode, you should read 0.6 to 0.7V.  With the test leads reversed, you should read open circuit.

For the 1N5236B Zener diode, do exactly the same test and you should get the same result.  Then do the test I described above to check that the Zener's reverse breakdown occurs at 7.5V.

Finally, there's a very small but non-zero chance that the resistor has failed.  You can check it with the resistance mode of the meter.  If it's within ~10% of 300Ω then it's fine.  If it's a very low resistance, then the resistor is pulling the tach to ground and there may not enough signal there for the G3X to see it.

I'll second Kevin's advice to check all of your wiring and the sensor.  As he said, you can use the AC voltage mode to check for the presence of tach output and if your meter has a frequency counter mode, that should give you a reading equal to the number of pulses per revolution times RPM (within the frequency limitations of the meter; it may not work at high RPM).