In really simple terms you only need fuel, spark and compression for it to fire. It sounds like you have fuel so check for spark on the front plug, you'll need to pull both so it's easy to spin it over. My MC16 exhibited similar problems if the battery was low, when the regulator failed, and when the CDI died. So worthwhile running a volt meter across the terminals to check the health of the electrical system as well if you can get it running.