Pull the spark plug and put your finger over the hole as you spin it over. That will tell you if you have compression. If I had to guess; the backfiring caused either a broken connecting rod or bent pushrod. If after the finger test you find a cylinder without, or very little, compression, pull the valve cover on that side and check valve function. if OK, pull head and check piston for proper motion. It is not a difficult repair...need a couple pullers for the flywheel and possibly gear cover to get at the connecting rod...will also need a ring compressor to re-install piston. It can be done in a few hours if you are handy and have the tools...pull the motor, remove shrouds, remove flywheel, remove stator, pull gear cover, remove head, remove connecting rod, install new rod, insert piston rod assembly into cylinder, bolt connecting rod to crankshaft, install gear cover, stator, flywheel, shroud and install engine. A small engine shop should run around 500 to do the job. fyi, Briggs has a 2 year warranty.