They make things called "screw extractors" that you can drive into an Allen head and get it out. I had to resort to one to get a stripped one out of my Harley drain plug once (Rounded off by the previous owner)

http://www.sears.com/craftsman-5-pc-scre...mp;blockType=G5

kirkusblog.com
2016 M3W John A Prestwich Special
1952 Ford 8N
1968 Honda CL175,and 2018 Ural Retro