1995 pick up won't start

I have a 1995 Isuzu pick-up, 4-cylinder (2.3l) with a 5-speed transmission with about 80K miles. A few months ago the engine started to stumble coming off of idle. Then, the check engine
light would come on as I'm driving with an associated loss of power. Removing my foot from the gas pedal and re-applying (usually) made the check engine light go off and the engine to regain power. The problem gradually became worse and the engine finally stopped running. Currently, it will not start. By shorting the two wires behind the left kick panel I was able to retrieve the error codes, 12 No tach signal to ECM 44 O2S lean 45 Oxygen sensor (rich condition indicated)
I believe that the original oxygen sensor is installed (I am the second owner of the truck, purchased it with 10K miles). However, I understand that a faulty oxygen sensor will not cause an engine to not start. So, I'm trying to understand how the computer gets the engine speed input signal. I have removed the distributor, but found nothing obviously amiss. I don't know how to properly troubleshoot the distributor, not having a wiring diagram of it. I assume that the engine speed signal must originate with the distributor, perhaps not.
Any help would be appreciated.
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