Not sure if this will help, but this sounds like what happened with an old lawn mower I had. That problem was that the ignition switch/control module went bad and it gave the motor bad signals causing it to be unsure if it had spark or not causing it to backfire and not run. Also, the valve clearance could of gone off. While highly unlikely, it is possible.
What I meant to say was the motor got unstable and out of time spark.