vanwert simplex furnace

Hi. I need to replace the burn chamber (or the liner in the chamber) in a 1979 vanwert simple furnace and have been told to replace the entire furnace. It works well, other than the fact that the liner is starting to crack and crumble. Anyone have any ideas or info? The oil company that services my furnace said they can't get the part...........the previous owner of our house told us several years ago when we bought the house that we could replace the liner ourselves, but we are not on speaking terms with him so can't get the info on how he did it before..............
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