You can tap the head.

Some tips to help keep shavings from falling in to a minimum:

Coat the tap with grease. Any aluminum they cut will
get trapped in the grease which will come out when you back the tap out.

Also, take a piece of cotton rope and soak it in motor oil.
Bring that cylinder up to just below TDC. Stuff the rope
down the plug hole in to the cylinder (be sure you have some springy fingers or some way to get it back out).
Fill the hole with the oil soaked rope as much as you can.
The oil on the cotton rope and the cotton rope will
help snatch any shavings that the grease didn't catch.
Pull the rope out gently and slow so you don't knock
any shavings off that it catches.

And after all is done and you get the rope back out,
take an air nozzle and blow out any residual debris that
may have stayed behind.