Hard remove Spark Plug

I had the same thing happen to me a few years ago. I snapped the plug off at the base with the plug wrench. I then used an easy out to remove the base, no harm done.

Whatever you do, don't damage the thread as it is not permitted to fit a helicoil in these heads.

Can you use a boroscope to look through the exhaust valve to see what it looks like on the inside?

It is easy to remove the cylinder head, could you then take it to a machine shop where they can extract the plug safely without damaging the head?

Try it while the motor is warm. Try shocking it by tapping the nut portion of the plug. Place a piece of heavy pipe or modified socket over the porcelain and onto the nut and tap with a hammer then remove. I’ve had success removing head bolts with this method. Reinstall with the thermal paste.