Will they cylinder head still be Usable after this scratch?
deep enough to catch a finger nail. Would a gasket sealer help or JB weld?

 

How do you know it's a 'scratch'? Why isn't it a crack? Either way the only thing to do is bring it in for a competent valve job to a good shop and pay the money otherwise ull be skayrooed. Hot tank clean and magnaflux. Even without this question that head is too knarley to just slap back on. Do it right get it overhauled. Then replace the lifters, and be sure to set preload before starting.