John:
I’ll agree with the earlier recommendations for Evan Designs. That’s where I’ve bought the LED’s for my GP9’s and 38’s. Evan gives you the option between a white LED and more of an incandescent version.
No need to drill. You just attach the LED behind the headlight lens. I use electrical tape but, two of my friends fashion brackets using styrene and hot glue those in such a manner to properly position the LED.
I only use an LED behind the lens on the front of an engine but, the process to add one to the rear would be the same.
Curt