Hot air furnaces are cheap enough and your heat load low enough at 40F interior temp that it's simply not worth the drive to pick up some oversized 17 year old "maybe". Your insulation levels are such that if it's reasonbly air-tight and glazed area minimal your mid-winter heat load at 40F interior temp probably won't break 25KBTU/hr even when it's -10F out. A brand new smallest-of-the line ~80-83% AFUE gas or propane hot air furnace is probably under a grand if you shop around, and even a small 30KBTU wall-furnace or radiant heater is probably enough to get you there.
Do a heat-loss calc on the place, figure out how much burner you need, and buy it. By the time a furnace is 20-25 years old you're looking at replacing contactors or motors in short years, possible corrosion issues with the heat exchangers, etc. They may or may not be worth repairing if they're already installed, it's freezing out, and you're desperate, but probably not worth re-commissioning in a new location.
If you like being actually comfortable in a 40F-50F shop room you might look for propane-fired radiant heaters mounted below the ceiling directed downward. Hot air furnaces suck from a creature comfort point of view, but ceiling mounted infrared is pretty decent. Gas models look like this:
Space-Ray (depicted above) and most other vendors make propane fired versions of their standard gas units. It's the right way to go in apps like this, IMHO.