Yea an old hanging gas furnace shouldn't have too many wires on it.
It most likely has a 24 volt transformer, it's simply a loop system, thru the thermostat, thru the high limit switch, to the gas valve.
Then on the fan wiring it should be 110 v and should run to a fan theormostat that kicks the fan off and on depending on the pendulum temp.
Well this all depends... is it gas or all electric heater???