[Red88Carrera]

If you apply 12v to X8 you will fry the diodes and possibly the coils.

Unplug the diode wire from both coils.
Turn the engine over and check for spark.

You said the starter wouldn't turn over. That could be a bad ground to the engine block, which would also cause the no-spark. It should be a large cable going from the battery negative, directly to a block mounting bolt. Yours may go to the frame, then to the block.

I build these engines all the time. I have 10 of them in my shop right now.
Again, it could be the plugs. One plug could have been bad for a long time, and the other one decided to crap out now. Most people can't even tell when this motor is hitting on only one cylinder. It will be low on power, but still sound almost the same.