How to add C wire to a very old 2-wire Lennox G16 Gas Furnace?
According to the wiring diagram for your system (i.e. Figure 2 in the linked PDF) -- the yellow wire on the 24VAC side is the C wire (it's the wire that goes to the compressor but not to the 'stat in the diagram).  Just incorporate the green wire from the thermostat cable into the nut with the yellow and green wires in it on the 24VAC side, and you'll be set!