Try shooting a liberal amount of WD40 or PB Blaster near the valve guides and slowly rotating the engine, while gently tapping on the tops of each valve with a rubber mallet. Then leave some to soak overnight. If still not freed up, I would pull the head to get a better look at what is going on. Chances are it's time for at least a valve job refresh and some new gaskets. If you do find catastrophic damage, then you can decide course of action from there before spending any more time or $.