Fluconazole can be used instead of clotrimazole http://www.antibioticsforfish.com/product-category/fluconazole/
Dosage for Fluconazole http://www.poultrydvm.com/condition/candidiasis
I’m not sure where you are so I don’t know what would be available in your area, but a human multi vitamin dissolved in water would work.
Great options if you can find them are rooster booster poultry cell, sav-a-chic, poultry nutri-drench, along with a b vitamin mix.
First the crop situation...
Does the crop feel hard or gooey?
It sounds like he has sour crop or a crop impaction, probably from whatever illness is going on, or the illness has cause some sort of malabsorption issue and he’s now suffering from a vitamin deficiency on top of the infection and crop problem.