Maybe you could use the current running through your high tension spark plug wires to illuminate a neon bulb by induction. The lighting of the neon bulbs could be sensed by a photonic (Cds, photo diode, etc.) which could trigger your sensor. Since there is no connection to the spark plug wire (induction) and your neon bulbs are not connected to the photonic, you would have a signal without a wired connection. That way you don't have to deal with the high current. I would drill a hole through a dowel with the neon bulb stuck in one end and the photonic device stuck in the other end to eliminate stray light from hitting the photocell.
It's just an idea...