I've never once, until today, heard of a spark plug blowing out from a Miata's head. Are you sure it wasn't simply not threaded properly, backed out then popped out? I'd at least check to see if it'll screw back in.
What engine is it? If it doesn't thread back in, the easiest / best thing to do is get another stock head. These engines are practically a dime a dozen, for the most part