2006 nissan xterra cylinder head

If the cylinder head is "damaged" -- cracked, valve issues, etc -- it depends on the "damage" to figure if it needs to be replaced or not. At the very minimum, the head should be checked by a professional machine shop who can check the surface to ensure it can be reused, as well as for any warpage and any valve issues. It makes no sense to reinstall a flawed head. No "sealer" is an option; repair the vehicle properly -- if in fact you can confirm there is a head gasket leak, which is certainly a diagnostic process in itself.