YOu need to get the maggots off. Not only will they kill living flesh, they release a toxin that will eventually lead to multiple organ failure. (We lost a Siberian Husky to complications from fly strike)
For flystrike in rabbits (I've never treated a chicken case, so I dunno if these things are chicken safe suggestions)
First pick off maggots. Tweezers help. Check any moist areas like the vent.
Next I spray with a flea spray for kittens.
Finally I give a shot of Ivomec for any internal ones I may miss. I have also squirted it into a hole that had maggots in it that I could not get out. (The rabbit lived)
Lastly, you'll need to check the chicken again in a few hours for new hatches, repeat every couple hours til you get them all.
Of course, the chicken/rabbit needs to be inside and poop free, if you leave it outside flies will continue to lay eggs on it. Downed or sick animals are at a high risk for fly strike.
In cases that I treated I had better than 75% survival rate. One I culled because the rabbits hide was peeling up and infected.