Wild backyard chickens huh, pretty rough shape and needs some care. It's really tough to say what this is because it is almost surely dry pox, but beyond that are the other complications that have caused it to get so bad. Her scratching probably caused it to worsen, introducing bacteria into the eyes and wounds.
First thing to do is to treat for worms. Almost certainly worms have contributed to her lowered immune system and allowed this to get like this. I hope you are familiar with how to deworm so that she can get the proper medicine within 24 hours. You would have to buy dewormer, probably at the same feed store.
Then she needs lots of clean water and continued feeding the day after the worming medicine has been administered. If you can deworm and get her some nutrition, the pox and swelling should clear up, as well as the scabbing. I have treated worse scabbing using coconut oil but it wont clear up until her immune system gets boosted. Vitamins and healthy food after the worms are gone. You can put soft pieces of fish, liver, meat etc in her mouth, but lack of grit is also going to become apparent if she doesnt eat on her own within a week or so.