Unfortunately, from what I can tell there does not seem to be any new LED that matches the footprint of the XRE. I’m going to try and replace it with a new R2-bin XRE, which should make a big improvement.
I believe you are correct about the ring being for heat-sinking. The reflector/finned heatsink part of the housing appears to press against there. Strangely, there was no evidence of thermal grease to properly pull heat away.
Here’s a pic of the LED board, which solders at a right angle to the driver board. The exposed ring around the LED connects to the housing to sink heat to the housing.
Oversize image of LED board 816-0007
I have been scouring the web for info on these old lights. https://www.google.com/amp/s/www.bikehugger.com/posts/light-motion-stella-300/amp/
Does anyone have an idea what LED is inside it? And can they be upgraded?