This is a classic valve adjustment issue. These engines have an extra mini lobe on the cam to partially open the exhaust valve when the engine is being turned over by the starter or rope. This is a compression release to make it easier to crank the engine. If the valve adjustment is too loose...
Vertical shaft tells me a push mower. A new engine will cost more than a new mower. Unless you just want to tinker with it. It is your time and money! That much gas in the crankcase tells me the needle valve in the carburetor is leaking gas right down the intake manifold and into the engine...
Not about spark plugs, but if hole strips, check the hole for depth and see if there are enough threads further down the hole. Then use a longer screw or bolt but use one long enough to reach the bottom. Carefully shorten the bolt until it will torque without hitting the bottom. You need to use...
It is getting enough gas to idle, but not enough to run fast. Dirty filter or cracked fuel pickup line. To test for cracked pickup line fill the fuel tank. If it runs good then, the line is cracked. When the fuel gets below the crack, it sucks air.