Mareks can cause tumors everywhere including the crop, all throughout the body, skin, and nerves. But her crop may be impacted. I would encourage you to give her plenty of water (electrolytes and vitamins would be good) and massage her crop several times to see if it empties. If it does start to empty, soft foods such as plain yogurt and chopped or soft egg can be introduced later. Does she normally have grit available? Here are some good articles about crop problems:
http://www.the-chicken-chick.com/2012/04/answers-from-chicken-vet-on-impacted.html
https://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments
http://www.chickenvet.co.uk/health-and-common-diseases/digestive-problems/index.aspx
https://poultrykeeper.com/digestive-system-problems/impacted-crop/
http://www.the-chicken-chick.com/2012/04/answers-from-chicken-vet-on-impacted.html
https://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments
http://www.chickenvet.co.uk/health-and-common-diseases/digestive-problems/index.aspx
https://poultrykeeper.com/digestive-system-problems/impacted-crop/
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