In the late 80's Kato introduced some nice models of the GP38-2 and the GP50 that were built on very similar mechanisms. These mechanisms were an improvement over what had been available up to that time and another stepping stone to get to what is available today. It is possible to install a decoder in these older engines and here's how to do it.
For this project I am using the TM3013 frame to modify a GP50. This frame would also work for 2nd run of the GP38-2 model. The TM3011 frame would be used for the 1st run of the GP38-2.
Remove the diode from the front end of the board. Using the Aztec frame as a guide, file a notch to match the slot on the left side of the frame.
On this particular installation I had used the Digitrax DZ123 decoder. Digitrax has replaced the DZ123 with the DZ125. Other decoders that would work include the TCS M1 or Z2, The Zimo MX621, or Lenz Silver Mini.
Red & Black = 7/8 inch
White & Gray = 1 - 7/8 inch
Orange = 2 - 3/8 inch
Yellow = 1 inch
Blue = 1/2 inch
In this example I used the original light front light bulb and connected the white wire directly. If an LED is used then put a 480 ohm resistor between the LED and the white wire. Do the same for the rear LED using the blue wire for the return path.