Re: Engine fire, bad burns.
Sounds like a stuck float flooded the motor. Not being able to turn the motor over suddenly is the first clue, especially after sitting overnight. Pulling the plug and clearing the cylinder of fuel is the usual way to deal with a flooded cylinder. I'm just suprized that it sparked enough to light the vapors. These dang things barely work as they are supposed to.
Conditions had to be just right for this to happen. Hope you heal fast and reconsider leaving the motorized bicycle scene.