You might need to do surgery. There are several guides on how to do it.
Here's one how-to I found. However, if you ask a vet to do it, I suggest you use stitches instead of glue. I found it here:
http://www.thepoultrysite.com/forums/showthread.php?t=6989
Chooks do indeed have nerves in their crops, so be aware of that during the surgery. And the incision will heal without stitches, but you can use suture or crazy glue if needed.
You'll need a sterile scalpel, soap, water, gauze or cotton balls, tweezers, crazyglue (superglue) that IS skin bonding OR a sterile needle and suture material, an empty cup to put goo in, a person to hold the bird, a towel, rubbing alcohol or betadine. Find an area on the crop that is relatively free of blood vessels- you'll be able to see them in the skin. The incision should be made about halfway down the crop- far enough down that you can reach the impaction but not at the very base of the crop. Cut or pluck some feathers from that area, wash the area thoroughly 3 times, rinsing in between. Dab the area with rubbing alcohol. Make sure bird is wrapped quietly in a towel and is held firmly by your helper. Generally the bird is very well behaved during this procedure... find the area that is more or less devoid of blood vessels, then, using your sterile scalpel, make an incision that is about 1 inch long. Use enough pressure on the scalpel to cut through the skin and thin muscle layer below. Once in the crop, quickly remove all the crud. When you are done and satisfied that the impaction is gone, give the bird about 10 cc of water in the crop to flush through any crud and give the bird a bit of water. If you have some, rinse the incision with saline solution. Now, without gluing yourself to your chicken, put a thin line of superglue on the incision and glue the edges of the muscle and skin together. Put the bird in an area where he won't be bothered, keep him quiet and warm and do NOT give the bird water or food. If you are worried about dehydration, you can give him subcutaneous fluids under his skin (on his back and where his wings meet his body). Inject STERILE saline, about 5 cc per side of the chicken, just under the skin until you form a little blister of fluid, The fluid will dissapate quickly. Repeat the subcutaneous fluids every 4-6 hours for the first 24 hours. The chicken should have nothing to eat or drink for 24 hours, then you can start to offer them fluids and pureed babyfood (squash, carrots, apples, pears etc). Check to make sure there is no weeping around the wound. You may choose to put him on antibiotics, such as baytril (10-15 mg/kg) if you suspect an infection (talk to your vet).
