Once you get the hard center dislodged, drill and tap the hole for a cast iron pipe plug- - - -most likely a 3/8 NPT. Install the plug with red LocTite to seal any potential coolant leaks, grind it flush with the top of the head, and then drill and tap it for the correct valve cover bolt (1/4-20). That's similar to the type of plug repair done for a crack in a head or block. No problems that way with heat expansion with a plug made from a steel bolt VS a cast iron head.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!