Spray some of yer favorite penetratingly elixir on the valve head through the spark plug hole multiple times during the day, then come back the next day and see if it will pop loose.
as mentioned, don't whack on it with a hammer because the compression release damage if so equipped with one.( I don't remember if a 10 hp has one.)
The valve adjustment compartment is a better place to tap/pry with a screwdriver.
If you understand the operation of the engine,
about 1/2 way down of the piston after the plug fires, should be safe as the compression release would not be holding the valve open, as it will be on the power stroke.
A
little tap tap on the valve head through the plug hole will not hurt it
