A valve that is stuck too tight to respond to penetrating oil and some gentle tapping probably needs more attention than you can give with the head in place. Even if it frees up somewhat, there's a danger of damage once you get the engine running. I'd suggest pulling the head and let a machine shop do a good cleanup and a valve regrind.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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