Here is he rub that most people seem to miss.
You test things to verify if they are working or not rather than making blind stabs in the dark.
There is an engine in the workshop with a complete piston & rod missing.
The customer had the mower for 5 years and thought it was a bit gutless.
It had been serviced 3 time by the vendor.
He was ill & his wife could not start it so she called my predecsor.
There was not a mark on the crankshaft, the engine left the factory with only one piston.
Briggs did the right thing & sent a replacement engine.
So the first thing to do is pull one plug out, ground the plug and try to start the engine.
Do it for both sides
Each side should start & run the same if both cylinders are working.
IF one side is not working then we do some more digging around to find out why it is not working.