Hi there. I have had two chickens with this in my 3.5 years as a chicken owner. One was sour crop, the other impacted/pendulous crop.
Both lived to lay many more eggs, and are still alive!
Both took several days to get better- the pendulous /impacted crop was a lot of work. This is what I did.
If there is a sour smell, you need to invert the chicken outside!- aim her face away from your shoes! If they are going to vomit, they will. It will pour out. I would count to 3. 1 Mississppi , 2 Mississippi, 3. (my Welsummer who had the impacted pendulous crop never vomitted, - my Brown leghorn vomitted A LOT.) They will not die or aspirate... you need to get the rot out. They may not barf on the first inversion. Massage the crop while inverting. Never for too long. 3 seconds.
Impacted/pendulous crop:
Brought her inside, set her own space up for her. Fresh water, grit.
Put her in my lap and firmly FIRMLY massaged her tennis ball sized crop. It was spring, and after a long hard winter, she had eaten a ton of roots and grasses. They were tangled in a ball, at least as big as a tennis ball.
I massaged for about 10-12 minutes, like 4 times a day. (10 days!!!) It would take a while before I felt like my massage was making progress. I would work on a little bit of the edge of the tennis ball. Then I would feel for a V shape in their upper chest. Maybe find a chicken diagram ? its the entrance to where they swallow. I would take a small piece I had worked away from th tennis ball and hold it firmly in the V.pushing gently. Eventually it disappears. (it leaves the crop to get swallowed I suppose?) The tennis ball got smaller every day. She only ate full fat high quality yogurt. Not mush progress was made from day 1-3, but then I recall it started to get better. When I thought she was ready- I made hand sewed a bra for her (like a sports bra out of old tensor bandage) (I am not good with a needle-) but it worked amazing. She wore it for about 2 weeks, then I took it off her and she has been great ever since. I made it really tight against her crop-and it went up fairly high so her crop was sort. of smushed against her chest, but in an upward fashion, not downward.Like a sports bra on a large chested woman. Anyways. Sounds like your chicken may not have a pendulous crop just yet. Start with a few attempts at inversion and vomitting, (if you feel she needs it)then move on to massage.
My chicken that needed to barf a lot- it would pour out of her mouth- you will know after 1 or 2 inversions... careful- they tend to shake their head while they do it.