Pin hole in cast iron head

I would drill the hole out to 1/8", not critical. Machine a piece of steel dowel [ or an old bolt ] with a slight taper to fit the hole. Push into hole by hand, & mark the point on the dowel where 1/16" sits above the hole. Cut the dowel at the 1/16" spot. Use Loctite, & punch/tap the dowel into position so that it is just below the adjoining surface.
I have used this method for plugging holes more times than I car to remember & it works everytime.....