I just had a girl with a head wound just like that a few months ago. We cleaned it really well with saline and then slathered it in triple antibiotic 3 times a day for the first 2 weeks and then twice a day. We also gave her Aqua-mox for a week since we already had it. We kept her in 2 extra large dog crates lined with news papers in the house so she could heal without the others picking at it.
Her wound healed in about a month. The feathers grew back over part, but not all of skin. I will warn you, it will look worse before it looks better. After a month, we put Hazel back outside, separate but near the others. After a couple of days, we allowed her best friend to move in with her. We kept them separate for everyone else for just over 3 weeks and then started to allow them to have supervised time with the rest of the flock.
While Hazel's wound completely healed, unfortunately, her original wound was caused by a fellow flock member (we never did find out who and no other chicken has had anything even slightly similar happen) and within a couple of weeks of re-integration, she was attacked again. We decided that we could not ask her to go through another month of healing and ended up putting her down
It takes a lot of time, but chickens can heal from a wound down to the muscle like that. Do you know who or what caused the wound?