Well thats a blast from the past. McGizmo used to make some custom modules for the M6 before I joined the forum, maybe around '04 and I have one that I bought second hand.
Buying a completed assembled Module would be a "drop-in" actually screw-in. There have been a few versions of the M6 over the years. The new ones have numbers on the side.
Mine just says Pelican M6 Lithium and works with the drop-in module.
The module is a complete replacement for the screw in bulb holder. There are still some for sale on dat2zip's website the Sandwich Shoppe
You'd need the module an LED, and Driver circuit, wire, Arctic alumina Adhesive, And soldering skills to put it together
You may find someone willing to sell an old one or someone could assemble it for you, but the price of parts will be more than a new current LED light.
Its about $60 for the parts