Over here in Oz we drill and tap the hole to 3/8 BSP. Then we screw a brass plug in with Loctite 262, and deck it and the head to clean up.

[edit] In the land of the free, you could use NPT instead.

That's very clever.  I will remember the technique for the future.  Thanks for sharing.

Sorry, these are out of sequence but these are Before pics of sunken plug and the gasket. For whatever reason, those brass plugs aren't tapered and they sink when there's a significant overheat situation and the joint is right there on the fire ring. Stupic. 

A pro shop will remove the old plug, taper the hole and press in a tapered plug then deck the head.

This DIY mod leaves the screw in the head (I've been using stainless) to prevent the plug from ever again sinking back down. The plug is pressed back up by screwing the screw in and it bottoms out and plug rides up the screw. Pull screw out a turn or two and mark it for cut off. Take it all the way out, cut it off, slot the head, put some loctite on it if you want, then thread it back in so it is countersunk and will keep the plug from sinking again.

Yes, you can get the plug cockeyed if you're not careful to get the hold drilled in the middle. But I've not had a repeat failure or coolant leak on any of the heads I've done this to. Not yet, anyway.

It's not perfect. It's not professional. But it's good enough in a pinch.

If you ever overheat your 1275 or can't seem to figure out why your head gasket replacement didn't last very long-- take a look at the brass plug that's between cyls 2&3 on your head casting of your 12G1316 and early 12G940 head. Also look at your gasket where the plug is.  Later 1275 12G940 heads use a steel core plug style and there's some with no plug, but not as common as the brass plug heads over here in the US.

Credit for the idea goes to Graham Reid of Heritage Garage, btw. He gave me this tip when I was in panic preparing for a LeMons event and couldn't find a head in my pile that was ready to bolt on. I've since done this "repair" 8-10 times.