That's true of a one-cylinder engine, but more than likely, KiwiEV's engine was a four cylinder, which would produce two ignition pulses per crankshaft revolution.A conventional 4 stroke piston engine will send a signal (a 12 volt signal IIRC) to the tach one time for every 2 rotations of the crank.
Bypassing the factory electronics is the first step towards building your own custom tachometer. Personally, I prefer to not install optical devices under the hood of a car. They will eventually get dirty and need to be disassembled and cleaned. When I was looking around for a way to put a tach on my diesel Golf (Rabbit here in the US), I came upon the idea of using the variable reluctance pickup from an 1980's Audi 5000 ABS wheel sensor. It's a nice, sealed stainless steel can with an integral wire attached. The wrecking yard guys are always fascinated by my crackpot projects, so they let me take it home without charging me.
My intent was to train the inductive pickup so that it sensed the teeth on the camshaft/injection pump sprockets on my engine.
Here's a photo of my test jig, one of the sprockets chucked up in the drill press, with a temporary wooden jig holding the sensor about 10mm from the spinning sprocket:
Once I had confirmed that the pickup was producing a strong signal from the sprocket using an oscilloscope, I fabricated a mount to hold the pickup in the proper location on my engine. Here it's fitted up on one of my spare engines:
The reason I'm bringing it up is that the inductive pickup could be pointed at nearly anything, a disc with holes drilled in it, a splined shaft, or (importantly), the teeth on an EV's flywheel (assuming it's still installed), with the pickup bracket pointing through the hole in the bell housing that the starter used to occupy. The inductive pickup can look directly into the teeth, or it can look across, no matter, it just needs to see an alternating pattern of ferrous material when rotation is occurring. Bolt heads would even work, but the more of them there are, the better low RPM accuracy you can expect.
There's more to this story, including calculations based on the number of teeth (dimples, holes, etc) that you are sensing, and a one integrated circuit electronic assembly project that can connect the VR pickup directly to your now available tachometer instrument movement. The end result is a tach that is dead-on accurate when calibrated, and is unaffected by water, dirt, heat, etc.
Interested in additional details?