Broken allen screw

Do you have a picture you could share?

Doesn't seem to be uploading for some reason.

See if you can see it on my builder log.

With the head of the screw broken off there should not be any tension on the bolt so it’s should come out easy.  I would first just try pushing around the perimeter of the bolt with a small screw driver or pick and see if you can just turn it out.  If that fails you may have to drill it and use an easyout.  Of course tape off everything except the bolt so no drillings get into valve rocker area. 

I suspect the bolt is going to have very little resistance to turning, so if you have to drill it, a standard right hand drill bit may screw the bolt in further until it bottoms.  You can buy easyout extractor sets that come with left hand drill bits, and in this case I would bet that just drilling it with a left hand bit would spin it out. If you have never used an easyout, there are videos on how to use them. Be sure to center punch the bolt before drilling and be careful not break off the easyout.  If you break it off then things get a lot harder. 

TA round wooden rod and using a hot glue gun to attach the rod will work well. Doing it this way won't get anything in the threads.

Or take something round and smaller than the screw shaft. Apply a TINY bit of superglue on the tip and touch the top of the screw head. After it sets in about 30 seconds try to unscrew it. Do not put to much superglue on the tip of your instrument because you don't want it going down into the threads. If you get super glue in the threads that may be an issue.