That is a very nice double rifle in what appears to be very nice original condition. Sad that the worms also found it to be so attractive. I'd agree that the horn buttplate should be replaced rather than wasting time and money on a repair.

There is a guy here named Joe Dobrzynski who often sells Buffalo Horn material for buttplates, grip caps, etc. I think he also sells horn material on Ebay. Here's a link to one of his recent For Sale ads here:

A true sign of mental illness is any gun owner who would vote for an Anti-Gunner like Joe Biden.