This fellow advertises Rolling Block parts on the Remington Collectors site-Lee C.Estabrook
lestabrook@suddenlink.net or 318-371-1299 in Louisiana.
Another option may be to repair the breech block by tig welding and remachining.This fellow advertises in the Single Shot Exchange-Pete Nagel
prnagel@frontier.com or 815-732-1249 in Oregon,Illinois.