How much damage can 6.5hp briggs mower have after running over tennis ball?

Just after buying a new Troy Built 6.5hp push behind mower w/bagger there was a tennis ball in the grass I did not see. After running over it the mower does nothing more than start & die. I pulled the fly wheel off, checked the key & it was not broken. I still need to check out where crank key is located. I read that you can disconnect the spark plug & with a glove move the blade back & forth to check if it is loose. I changed the spark plug & it still starts & dies. The only other thing I can think of is the timing of the electronic ignition from the magneto may not be sparking at the right moment of the piston position. Can anybody give me advise on how to check the key on the crank or how to check the
timing? Can anybody tell me what kind of damage they have had from pushing there mower over a tennis ball?