Large or bantam chicken?
If large, ask your vet for a small animal force feeding tube and a large syringe. The tube is about a foot long and a quarter inch thick. Buy baby parrot formula and mix it according to its directions, but also add a small pinch of electrolytes to help prevent diarrhea (a liquid diet of baby parrot food will cause diarrhea, electrolytes will at least help lessen the damage done). Don't feed oatmeal. It may look tasty but it has virtually no nutritive value for a chicken. Once you get the baby parrot food of a good consistency and temperature (slightly warm, but not hot), fill the syringe. You will need someone to help with the next step. Have a friend hold the bird and keep its neck extended straightly. Oil the tube with olive oil and slide it down the bird's throat (don't worry about accidentally inserting it in the trachea, most tubes are too large to fit). Idealy, you want the food to go directly into the crop, but as long as you're far past the trachea you're ok. Guesstimate the distance from the mouth to the crop. If it won't go any further, stop. Then push the plunger down fast and viola, the bird has been fed. Don't overfill the crop, but don't underfill. About half full is good (slight bulge on the chest). Do this at LEAST twice daily, 3-4 times is better.
Good luck! I kept a bird alive for two months doing this. After that I had to put him down because I just didn't have the time to dedicate to him. He would have survived though.