Then I would use the horn as a mold and put layers of fiberglass on it to strengthen the horn (inside and out)
Leather paper can then be looked for, a bit of gold painting on the inside and darkbrown paint on the outside...
Buy a good Meltrope reproducer on ebay or even an Orchorsol.(i have a spare one if you like)
I just fixed my sagging horn of a emginn senior yesterday.
Took it in the shower to soften it....Now its as straight as never before and dry after 12 hours.
I guess in the 80 years of existing someone using the gramophone outside leaving it in the rain for a brief moment is enough to get the horn sagged.
I know it sounds stupid but I left my convertible cars open many times in the rain
I just need to do some minor restoration and have to put new leather paper on it...
You can't bake an omelet without braking some eggs !
Just go for it!