Leave it alone if its got proper resistance.
better to be risk adverse than have to tap out a broken off GP. I've seen a head at a machine shop where the threads got messed up at a shop where someone tried unsuccessfully to get out a broken off plug on a 606 head. They are bad in that respect.

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!