In Suburban Furnaces the NT-XX number indicates output in KBTU. An NT-12 is 12,000 btu and is the smallest and more than enough for most Scamp sized rigs.
But do beware, 20 y.o. furnaces out of the Junque yard can be death traps with leaky gaskets and defective combustion chambers. A
furnace is not a good place to learn-as-you-go. If you do install one, get it checked for carbon monoxide
leaks.