I am sorry, I am only trying to help so please do not take offense, but ducks absolutely do *not* have crops. I don't care where you read that or even if you heard that from a vet. They do not. I am a poultry science major and have necropsied probably at least a hundred of them (as well as other species including several birds that had proventricular dilation disease so I have seen a lot of digestive tracts) Please keep in mind, Avian medicine is a very small field that is really just coming into its own. There is a lot of incorrect information out there as this is a very specialized field that has not gotten a lot of attention in the past.
Again, I am trying to help, not offend you. I was just curious why you thought the bird had a malformed crop? Is it because you can feel something or is something visible like a lump? If this is why you suspected a malformed "crop", it could very well be that the bird has an impaction. If so, there may be a way to fix it.
I have never heard of a bird with an impaction living for so long with no other symptoms other then stunted growth. Usually they die rather quickly if the impaction is not removed, either manually or surgically. That is why I would suspect something genetic.
A key bit of information though before I give any more advice (if you want it and I haven't irritated you by insisting ducks have no crops, LOL) is why you think it has a malformed crop. If you want to give more information, please do and either way, I hope your baby pulls through!