Sour crop usually smells putrid and the crop is distended. I have (as per my poultry books) emptied the crop by turning th chicken upside down (so the crop's contents fall out instead of flowing into the lungs) and gently massage for no longer than 30 seconds until the contents come out. Sometimes this has to been done a few times with rests. Then giving some apple cider vinegar for a few days will help pH balance.
But is it wise to do one that's so sick?