Ahem
Don't actually know if there is an actual "Go-To" guy for a Rossi Coach. The guns haven't been popular enough for CAS to garner a great following and Specialists. However:
Were I a betting sort, I'd contact Nate Kiowa Jones at Stevesgunz. I've personally retired from smithing and don't know if Steve has retired or not but Steve is a superb smith and dam'd honest to boot.