I don't know about Hydro Flames, but if I read the prevailing grok on old furnaces in the AS community correctly- old furnaces are hard to find parts for, get serviced and present a CO poisoning hazard. IMHO replacing old appliances with new does not detract from the value of the coach but adds to it. The possible exception being an vintage fridge or freestanding heater which have distinctively different appearance from new units. If you are a fan of aluminum masochism
and want to keep it pure there are several sources for old appliance parts in Elkhardt Indiana, but I can't remember their names.