How to read a high voltage signal with Arduino (wirelessly)
When I was a kid, I used to hold neon bulbs next to a spark plug wire. The bulb's leads in my hand. The leaking electrical field around the spark plug wire would be enough to light the bulb's gas. You might try this and connect the bulb's leads to the bike's ground (chassis). Now, use black heat shrink tubing and construct a device that optically couples the neon bulb with a photo diode connected to an Arduino. You may need to distance the photo diode and Arduino form the spark plug wire using optical fiber. I imaging the electrical noise next to the spark plug wire will cause all kinds of problems with the photo diode and Arduino.