Somewhere buried in the archives should be a description of how I modified Mini-Mags to take PR2 type LED lamps.
I put the lamp in from above and turn on the light by tightening the head rather than loosening.
The short version is this:
Dissassemble the light and remove the switch assembly, remove the anodizing from the top edge of the battery compartment. Place a suitably sized conductive washer (if necessary) over the top of the battery compartment and slide the Epieon or other PR2 style blub in from the top. You will need to tape around the side of the bottom of the bulb to prevent electrical contact with the inside of the battery tube.
Cut off the portion of the tailcap above the threads and bore the hole for the spring deeper or switch to a spring that fits into the hole for the spare Mag lamp. Assemble the tailcap and battery tube with batteries inside (positive up as usual) then insert and push down on the LED bulb at the top of the battery tube. The flange of the bulb should contact the washer which should contact the top of the battery tube and the LED should light. Release the pressure and the LED should go out.
At this point you may find you need more washers below the flange. Reaming and grinding washers to size is a pain. I used bronze bearing washers from the hardware store, more expensive but very close on the ID and OD.
Next, ream out the hole in the reflector and carefully reduce the height of the reflector from the bottom until the focus is satisfactory. Or put in your optic. You may need/may want to plan for a stiff thin washer at the bottom of the reflector to help spread the force because it is the thing that pushes down on the collar of the emitter that pushes the flange/washers down on the top of the battery tube to complete the circuit.
I've done quite a few of these hosting "Night Star" lamps and "Pro Series" Luxeon lamps. Done carefully, it's just a bit longer than a stock Mini-Mag and the head still covers the o-ring with a thread or so to spare.
Hope this is clear enough to be usefull.