You don't mess with furnaces that old. I had a furnace replaced that was pushing 30 with only occasional use in our condo because the duct in the basement had been chewed out by a squirrel, and, come to find out, it had been running with a cracked exchanger. We had no idea how long it had been cracked and we didn't have an Monoxide sensors in the house. After a certain age, you bite the bullet and get a new furnace.