The second half of the circuit is simply a schmitt trigger to reduce the chances of the tach seeing a glitch. Op amp U3b compares the signal from the input filter to a voltage reference provided by D12, and modified by feedback via U3a. The resistors' values have been carefully chosen to be easily found and also to track varying battery voltages sanely. But essentially, R27 and R30 set the centre of the trigger voltage while R29 sets the envelope.
The schmitt trigger is also set up to invert the signal, which presents the tachometer with a signal which looks more like a typical 4-stroke battery ignition signal.

LM358 was chosen because it's durable and easily found. It is a dual version of the quad op amp LM324.

The current through D12 is on the order of 3 or 4 ma, so any type of 3v zener will do fine here.