When I received the A-B-A set of FA-2 engines, I had just a day or two to get them ready for a big local train show. I decided to use some Digitrax decoders left over from my N-scale days in these engines. I used the Digitrax DZ-143 in one A-unit and a DZ-121 in the other A-unit (the only two decoders I had available). They are rated for 1.2 amps continuous and 2 amps peak, so that should be sufficient for the motors and gears in the American Models' engines. I have found several web pages stating that the stall current of these motors is 0.7amps. Some web sites I have visited warn about replacing the light bulb used in the A-units when converting the engines to DCC, but I found a small circuit board and an LED already installed in my A-units. The conversion is quite straight-forward. There are four screws that attach the shell to the frame. Once those are removed, the shell slips out. There is a small circuit board attached to the frame with some double-sided tape. The headlight wires are connected to that board using a 2-pin plug. One of mine popped off easily and the other unit required a pair of pliers and some force to remove. The LED is glued to the shell using hot glue. If you need to replace the headlight LED, use a hot glue gun to melt the original glue.
To actually install the decoder, I removed the circuit board from engine, and unsoldered the wires from the motor's leads. You need to isolate the motor from the two pickups of the frame (one side connects to the wipers, and the other one to the frame itself). I then connected the black wire of the decoder to the black wires of the frame, and did the same thing with the red wires. The gray and orange wires of the decoder go to the motor's leads. The photo below is of the trailing A-unit which, for this installation, did not have the LED headlight wired up. However, the white and blue wires of the decoder can be routed to the LED, with the appropriate current-limiting resistor (which is what I did on the other A-unit).