My Dad has a Miller furnace Mod# MMHA-075AAHR-01, Ser# MMH9104-02843. I see that it's a Nordyne unit now. It has a cracked burner chamber so I need to replace it. It's in a in a 1970's single wide, in a tight spot, LOL, which I'm sure is common in mobile homes. What would be the easiest/smallest? re...