Lapping both valves into their seats, and then setting the clearances, seems like a good idea. One possibility is that the mower had carbon in its muffler (maybe a second-hand muffler had just been fitted) and the previous owner took fright at the burning carbon and threw the mower away. If so, it is your good fortune, now that you've burned it all out of the muffler. That doesn't really fit the fireball symptom though - a burned exhaust valve makes more sense. You'll know as soon as you look at the valve seat.
