A couple of turns of 1 mm² wire around one of the sparkplug leads will work fine.
Leave one end open and the other end goes to your BC547.
Bring that to the base of a BC 547. fit a base resistor 47k. and a protection diode + resistor to negative to protect your base from overvoltage.
A bit of trial and error may be needed, see what works best.
Than amplify signal to whatever circuit you want to drive.
I used it to drive a LED and a pulsecounter in my old Ford Falcon.