Mind that, if the spark gap fires regularly, it'll sap voltage from the coil and likely cause misfires.
What's D4 for? D5 will conduct in the forward direction as well, and the LED won't mind a volt or two in reverse.
If you have 12V available, have you considered this circuit?
- Remove SPARK_GAP
- Connect 1kV diode (ES1J say?) in series with X1-1 (pointing left)
- Connect pullup from +12V to left of R7
- Move D5 to the left side of R7
- Add capacitor from R7 to GND (left and/or right side) as needed for filtering
- Adjust R7 value, and replace D4 with a resistor
- Remove OK1; clamp diodes and resistor dividers will suffice to get a clean logic level.
Assuming the ignition coil is wired as normal, i.e., one side to +12V (or a current limiting resistor), other side to points or ECU output switch, to ground.
If it's ground referenced instead (switched to +12V), flip the input diode, and move the pull-up from +12V to GND (pulling down).
Note that ignition is timed to turn-off, coincident with the voltage spike. I suppose you could just as well use a large value resistor divider to sense this directly -- with say a 100kohm resistor, a few hundred volts is only a few mA, easily clamped with diodes.
Tim