Well if it's on an outside wall it might not be a problem.
When we bought our place some 24 or so years ago, the guy from NSP as it was called then, checked out the furnace and said that though it wasn't the most efficient, that it would probably not break down and that at the time, it would take close to 20 years for any savings to be made as the new furnace and ducts would have been about $6000. So we kept it. I've been able to fix it myself. It does keep the place warm, winters are about $200/month for gas. It is huge, but looks kind of cool. I'd like a new one someday, but man.. $$$