Can I enter my road racing car in a rally?
Short answer: Slim-to-no chance, unless the rally is being held at a special venue and is explicitly allowing road racing vehicles.
Long answer: The safety cage rules are dramatically different between rally and other forms of motorsport. The biggest differences come down to the tubing diameter and the number of attachments of the cage to the body of the car. Rally cars require thicker tubing than road racing cars, so any normal road racing car will have a cage that does not meet the minimum rally specifications. And within most road racing in the US, the philosophy is that safety cages are supposed to be for safety only, and not for stiffening the chassis, and thus they are often limited to only six points of attachment between the cage and the shell of the car. Contrasting that to a rally car, which might easily have twenty connections to the shell.
If you are in the situation of already owning a race car with a logbook, you should reach out to the closest scrutineer to discuss your options. At a minimum, some amount of cage work may be required.