|Typically, engine kickback is caused by improper ignition (spark) timing. Timing problems can be caused by striking an object (i.e. lawnmower blade hitting a hard object) while the engine is running, resulting in a sheared flywheel key and/or bent crankshaft. Timing problems are also the cause of hard start or no start concerns and is the cause of serious performance problems.
Your best option may be to have the engine examined by a Briggs & Stratton Authorized Dealer. Your local dealer can determine if the crankshaft has become bent, and if so, if it can be replaced. In some cases a short block is necessary.