If you put a dremel to a toroidal ferrite core and then put a HALL sensor in the gap you can pick up the current in the wire to the ignition coil.
This is probably the most reliable method.
No need to interrupt or change the existing wiring in any way.
No troubles with for example mosture or accidentally shorted wires.
You do not want to get stranded in the middle of who-knows-where because your tacho meter addition is messing with your ignition's ability to generate proper sparks.
Another way is to make a capacitive pickup.
You can wrap a wire around the primary or secundary ignition coil wires a few times (no electrical contact) and fix it with tape or shrink wrap.
The 80V or more spikes are mostly irrelevant, as long as your circuit can withstand them.
Start with a 100k resistor in series with your wire and a few fast TVS diodes from your power supply.
That should take care of clamping down those transients, especilally if combined with a small inductor in series with your resistor.