Welcome to the site.
I'm sure others will chime in with their favorite tricks, but what I'd do first would be to thoroughly clean the inside of the case and the horn, inside and out.
Then, I'd put the horn back in the case, isolate the whole assembly so no one will accidentally move it, and place an open container of baking soda inside the case. Let it sit for a week. You may want to change the baking soda, too, during that period.
A complete overhaul may also serve to render the horn clean.
I'm not promising anything, but that may do the trick. DAVE