https://poultrydvm.com/drugs/sulfadimethoxine
But I would treat the crop issue first as it might be the only reason for her weakness. Or do her droppings look bloody?
Inspect her for lice and poultry mites as they can leave a chicken anemic in days.
In case the Amprolium/Corid is not working, you could use a sulfa drug like sulfadimidine/trimethoprim which will usually make them feel better already the second day of treatment.
Feed her some scrambled eggs...