I've got a 1921 craftsman bungalow with hot water, radiator heat. It's an open gravity system with the expansion tank in the attic. The expansion tank has extensive corrosion and no longer holds water.

I don't find any information ****** about replacing open system expansion tanks. Is there a place I can find replacement tanks or would I need to have one fabricated? Is there a compelling reason to convert it to a closed system with an expansion tank in the basement rafters instead?