If the affected area can't be draw filed out and rust blued, then I would leave it as is. I'd hate to mess one that sounds so nice myself, other than careful repairs. The forearm tip horn is easy. I have pieces of horn I got at the knife show, and I carefully whittle and sand them down to make fillers, then use colored Acraglass Gel to glue them in where it's missing.
Marlin lever actions 1870's-WWI, Ballards, and single shot rifles!