I began installing new valve stem seals on my 1972 220D today and found a surprise waiting for me under the valve cover.

One of the head bolts was broken off at the socket head and was just bouncing around in there along with the washer.

The car runs well with no signs of a head gasket failure.

I plan to finish replacing the valve stem seals today and button it back up, but what are my options regarding the broken head bolt?