Can be that valves are out adjustment as mention; unless, you used the old school method. OR the camshaft ACR is failing to operate properly as it sounds like a compression lockup with you removing the spark plug.
Not a whole of experience with the 09P series as push mowers here are becoming a rare item in my shop due the low replacement costs. Especially when the replacement engine list for $282 and a complete mower is around $150.
