Looks like a very nice horn.
I assume it is empty now. Is that correct?
It needs to not leak. That's the important detail. You can usually just look over it to search for leaks. If you are not satisfied with a visual inspection, you can blow into it and then feel around and see if you feel air coming out. If it has small holes, they can be repaired.
Don't pour anything into it. Don't pour oil or water into it. You do not need to clean the inside.