Or get them here (even though it says they won't work on a drum style tail light, they are the same ones that Bratton's sells which do work on a drum):
https://www.snydersantiqueauto.com/led-lamp-kits
Use some of the bubble wrap with the very small diameter bubbles wadded up and inserted behind the green printed circuit boards to hold the LEDs up tight to the back side of the lens and they work great!