I gather it's the Allen Key recess in the head that's been rounded out.
You could try grinding a piece of flat steel, like a screwdriver only with a wedge taper and sharp edges slightly over-size to the recess.
Tap it in and turn with pliers. Once you break the "seal" it should come out with finger pressure.
You can buy replacements from any m/c mechanic for around $2 each.
The alternative is drilling the centre and applying an "Ezyout". They work fine, but screws are notoriously hard to drill and Ezyouts have been known to snap off DAMHIK