Plug sliver in head

Knucklehead that worked on my student’s plane before him used a combo of excessive heat transfer compound to the tip of the plug and anti seize which combined to actually shear half the threads at the tip of one plug off when removing it. Sadly were not able to grab it and it fell into the head. Tried high strength vacuum with thin extension and compressed air. Neither encourage it to come out. We assume this thin aluminum plug threads sliver can’t be trusted to safely exit through the exhaust and we need to remove the head to remove it? Any other ideas? 
Rest of that debris not circled is the pieces of compound that entered when chasing the threads to remove all the extra built up compound/anti seize, to be able to get the plug back in. 
sigh. TIA