Euthanize. there is not enough skin to pull that one together to close it, and it suffers already. Sorry. Keep closer watch on flock health, I know it is difficult sometimes. I am not a vet!! I will describe what I would try if my injured bird is valuable. This might take a couple hours to attempt, and your bird might die anyway from blood loss and stress. Before its organs dry up
! If you were going to process a cockerel today for your table, you might have some skin for transplantation that might work, but then again it would be a huge trick for the new patch to gain enough circulation from the surrounding skin that it would survive that big a piece. If you do have a bird to sacrifice for new skin, and decide to attempt it yourself, have one person debride the injured bird of its yukky dead skin edges while the other person, as quickly as possible, cuts the new patch off the table bird, take from same position on body, immediately after killing it, and attach the new transplant to live (probably now bleeding) tissue. You've already used non-sterile water? Use povidone iodine on the organs before closing and on the outside afterwards. Then antibiotic ointment on the joins. If you've not sutured before, either get a pro or make sure you don't tie too tight to cut off circulation anywhere but still close. Black thread so you can see it in ten days to remove it. Sew quill-to-quill every 2 cm for strength. My general advice for all injuries on chickens only for covering the job is don't use gauze, use cotton stretched, then lots of narrow sticky masking tape (not bandage tape) all over to hold it on. Of course sticking it to a wet chicken is impossible so wrap it all around the body. Not too tight. Daily or at least every other day cut it off, check it, and re-bandage. Give Poultry Cell in crop with eyedropper. Add a kind old buddy bird within sight. Keep patient quiet, clean, and warm. If the new patch does not take all the way, remove what died and pull the live together. Sew only live to live. Check the rest of your birds! good luck.