Unless you have the tools and the experience in removing broken plugs, either in situ, or on the bench, then the best method is to remove the head and take it to a competent automotive machine shop. They'll pull the broken part out, and possibly install a time-sert or equivalent plug repair insert. When they are done, the repaired head will be just like new. They might also be advised to chase the other three plug holes. And of course you'll use Never-seeze when you install the new plugs.
These things happen, all the time. You don't need a new head, and no amount of penetrant will fix it. In fact messing about in the dark will only make the work of the machine shop more difficult.