OK ,
Scratch one off the list but only if you have checked the underside of the manifold.
Either saturate it with WD 40 or similar , spin the engine then check for WD40 on the spark plug
OR pull it off turn it over and do a visual check they have been know to split the entire length underneath.
If it tests sound ( don't over tighten the tiny bolts ) remove the kill wire from the coil then attempt to start the mower again.
If it starts & runs ( note you will have to pull the plug lead off to stop it ) then your problem is the kill switch.
If there is no difference they you have crossed 2 items off the check list.
Next is to pull the carb off, turn it sideways and blow air through it from the filter side.
If no fuel comes out then the carb is blocked
If fuel blows out then that leaves you with timing & valves.
Timing is easier to check than valves so pull the blower housing off , remove the bolt / nut and the starting cup.
Under there you should be able to see the 2 timing key slots.
They must line up together to make up a perfect square .
You can do this earlier on while the blower housing is off.
The last thing to check is the valves .
The valve chest resides behind the muffler.
Usually on a side valve the exhaust seat burns away so the exhaust valve does not fully close.
If they barely move then the cam is worn out.
If you have a red flashing spark tester you could hook it up to the mower before you pull it apart and check that it continues to spark even when the engine has stopped firing.
While the flywheel is rotating fast enough and the mower is in the on position you should get a spark.
Some times a coil will fire a few times then break down & not spark.
