Small Engines (Lawn Mowers, etc.): B&S issues, head gasket, spark tester

Question
I have a 4HP Briggs, model 10A902 on my MTD push mower.  The thing ran fine a couple of months ago but now i pull it and get nothin!  In my attempt to revive it, I cleaned and rebuilt the carb, replaced the ignition, replaced the spark plug, replaced flywheel key, checked piston (it moves fine, no scoring on cylinder walls), checked valves (they open fine and close nicely), replaced the head gasket, and finally i changed the oil.  I put fresh gas in the tank, primed it and when i pulled it i get nothin, just the occasional slight backfire.  Now I am very perplexed as to what is wrong or what I should do.  Any advise would help, thanks.  

Answer
Sometimes the engine shut-off cable stretches preventing ignition.  Remove the spark plug, hold the engine shut-off lever and check for spark.  A spark tester is the best tool to use but you can remove the plug, re-connect the plug to the plug wire and pull the string with the end of the spark plug touches a metal part on the engine.

Let me know if you see spark.
Eric