Oh, hun... that doesn't look good at all. We've had run-ins with those nasty opposums too.... I am so sorry.
I have seen with my birds that chickens have an incredible ability to heal. (Had one with a huge chunk taken out of her flank and I cleaned her up (used squirt bottle with warm sailine water and flushed the wound clean, then I smeared it with antibiotic ointment and put her in solitary for the night. She still had a spark in her eye in the morning, so I fed her some garlic mashed in scrambled eggs...(antibiotic/antibacterial properties in garlic) and, though I wasn't too hopeful, she didn't seem worse on day two, so I thought I'd give her a few more days before making a final determination. Every day she seemed a little perkier... within about 2 weeks, she was laying again...(an egg a day) within two months, the 2 1/2 x2 1/2 inch chunk was not visible at all. I was amazed.
From your picture, though, it looks as though the birds CROP (sack that food goes to after chicken eats swallows it) has been torn open as well. I have no idea if stitching the edges of the crop closed would "repair" it (I have no idea if it has the ability to regrow and seal itself as a flesh wound does) or if it would become leaky and be a constant source of re-infection for her. If you are going to attempt to stitch her up, you'd need to address the torn crop first, or you'll have all sorts of infection issues/thrush issues etc. Unfortunately, my instinct is that she won't recover from a torn crop injury... every time she eats/drinks, that crop gets stretched... and it would have to be leak tight to keep food/liquid from entering her body cavity instead of staying in her digestive tract.
I really hope it's not her crop!
Good luck!
