Broken bolt in stem

Two part epoxy might do the trick, not sure how tight the bolt is though . . . . ?

I would put an Allen key into the bolt and then pack out the space left where the rest of the hex head has broken away . . . should provide enough support to turn.

You could even cut an old spare hex head up to fill the space.

used a two part epoxy (superglue brand) on my rounded allen bolt, left overnight to cure. did not bond well enough. bike in shop having new headset fitted. and for shame a new bolt to replace my fucking spanner hands mistake.