ignition timing light trigger current
I have used such a circuit for my ostroboshcope. For picking up the signal you will have to use a pick up coil. As it is not to be found ready made you will have to make your own. First lets make a bobbin for pick up coil. Take a piece of cardboard about one inch in width and wrap it very loosely around the ignition cable and paste the end of the cardboard strip to the wrapped cardboard with glue. Now the cylindrical bobbin is ready and wind about 100 turns around this bobbin using 34SWG enamelled copper wire. Solder leads of the coil to a screen wire and connects these leads to your in put directly. When you want to check an ignition wire, remove the spark plug top 
, insert the ignition wire through the bobbin of the pick up coil and fix the spark plug top. Thats all . And change the resistor at input to 1000 ohms.