It is not really that hard. Should take a day or so. Most people put in a Suburban NT35. You have to do a little fabrication to get the furnace to line up with the exterior intake exhaust holes in the wall and to connect up with the ducts. $400 to $600 for a new furnace. There are multiple old posts with instructions in the archives.
Another thing about the old furnace is it is very difficult, if not impossible, to find someone to work on them and parts are a problem too.
An easier solution is a wall mounted catalytic propane heater. This type of heat has no moving parts, is silent, is radiant heat so it heats people and objects facing it in a localized area, is less expensive, easiier to install, and needs ventilation. But if you just need a little heat in the morning or evening may be useful.
Sail on silver girl. Sail on by. Your time has come to shine.