You may be able to add some electrolytes to your cats fluid intake. Ask the vet to make sure there aren't any contradictions due to the impact trauma.
Sounds expensive, but isn't - usually less than $6 for a bottle that will last a cat a long time. Any drugstore and a lot of grocery stores sell electrolyte solutions in the baby care area. One brand name is "Pedialyte" but you can buy the generic. Just make 100% SURE that you buy the PLAIN UNFLAVORED CLEAR kind. While at the drugstore, ask the pharmacist for an oral syringe. (Most just give them away.) Mix filtered water with the electrolyte solution at about 4:1 - 4 parts water, 1 part solution - and give your cat at least 10cc's of this mixture every hour (that you can.)
As your cat gets stronger, you can increase the ratio. It has helped me nurse numerous cats back to health.
Good luck with your kitty and let us know how things go. =)