I hooked up my multimeter to the signal wire for the tachometer and it seemed to pulse with 1,5 to 1,8 kV or so. Is there a (fairly) simple circuit that can convert this signal into arduino-readable format? or should I go for an alternative like a cable wrapped around my ignition cable?
See if you can find out more detail about what sort of signal that is. If you really do mean KV (kilovolts) then that doesn't sound like a tacho output. You need to find out whether it's a digital signal or just a wire connected to something in the ignition circuit. If it's digital, you need to know how the RPM is encoded on it. If it's just a wired connection you need to know what it's connected to - the coil LT -ve is what's usually used for an analog tacho output but you need to know what you're dealing with before you can decide how to deal with it. As I said before, in my experience inductive pickups suck and I wouldn't recommend trying to pick up the spark signal inductively.
Could you point out what the other things are that the yellow wire goes to?