How hard would it be to convert one signal to another?
For example, I'm using the RPM sensor that Manzanita sells because it was specifically designed to work for the Zilla for overspeed warning. What would be the easiest way to take this signal and convert it to a suitable signal that the original tachometer takes? i.e. electrically instead of mechanically (two sensors). I stupidly did not keep my original RPM sensor from the distributor.
Your OEM tach probably expects one pulse per ignition event. On a four-stroke engine you get half as many ignition events per revolution as you have cylinders, eg. on a four-cylinder you get two pulses per revolution.
Depending on how the Manzanita sensor is made (and what the Zilla expects) you may only get one pulse per revolution. While it's easy to divide a signal down (i.e. turn two pulses per revolution into one pulse per rev), it's tricky to go the other way, especially if the incoming pulse train is narrow (which it probably is, if it's generated by a magnet or Hall sensor) and if the RPM range is broad (100 to 5000 RPM is a span of 50 to 1, very broad).
So, while it's nice to have a trick electronic solution, it's pragmatic and a lot easier to avoid unexpected issues, to have two sensors, each optimized for the task at hand.
The other option is to create a single sensor that puts out two pulses per rev to drive your tach, and then divide it down to drive the Zilla.
Note that if your sensor is generating two pulses per rev, then using that to drive your tach might be as simple as wiring in a buffer transistor (to keep the tach input from contaminating the Zilla's input expectations) and a pullup resistor.
Hope that helps... if you can provide more details on what your system has then I can give you more specific pointers.