Pull the valve cover and check the valve clearances. The engine not wanting to crank over may be due to valves needing adjustment to restore function of compression release. Don't worry about the flywheel key shearing, that is not something likely to occur on a mower that has a belt driven deck, The concern over shearing a flywheel key by hitting something and stopping the blades is only true in push mowers where the blade mounts directly to the crankshaft. Belt driven blades can't stop the crankshaft quick enough to shear a flywheel key. The puzzling part of the description is how the exhaust seems to have gotten louder just prior to backfiring. Start with the valves.

One end of spring is connected to a stationary bar and other end is connected to belt pulley that the backside of belt rides on and is tensioned by the deck release hook if it is a 48" or larger deck. This pully looks like a double pulley and is flatter looking. Look to be sure the spring has a hooked area at both ends. If not then one end is broken off and belt can't be tentioned and you need new spring. Tim

"perhaps your kill wire is cut, or disconnected." Agree. You really need to post the make and model number of your engine when asking for trouble shooting. Assuming that it is a B&S flat head single, there will be a junction terminal on the side of the throttle box, where the throttle cable goes to the engine. That terminal is where all the "Kill" wires connect, from the switch, the safety switches and the wire from the coil under the cooling shroud which is the "kill" wire from the coil. Touching a screw driver or jumper wire to this terminal and to a ground should shut off the engine UNLESS the Kill wire from the coil is disconnected or broken. That is a starting point. As for starting the engine with the switch "Off", just use a jumper wire from the terminal where the cable from the battery attaches to the starter solenoid and to the small terminal on the side of the starter solenoid. Walt Conner

No, a loose or dirty connection somewhere. Inspect the power and ground leads well for corrosion inside the twists and replace is necessary or clean if possible. Something is causing enough resistance that power isn't getting to the starter from the battery. The charger/starter pushes enough amps to overcome the resistance. Backtrack from that connection you just cleaned and snug all other connections and clean them if necessary. Do this with the battery disconnected of course.