That said there are lots and lots of potential options. Really depends on how much effort & time you want to invest. How much customisation and what your end goal is.
The MR16 mod linked above is really simple, quick to do and only requires a small amount of soldering. None inside the torch and can be completely reversible. And it still runs on D cells. The down side is, it’s only 260 lumens at best and very floody. Makes for a nice in the house torch, but not enough beam distance to use across the fields IMO.
Other really easy options are the bulb style drop in’s.
Review here:
These aren’t the cheapest option, but are pretty cheap. And again won’t make a lumen monster. But will totally transform an incan Mag and make them very useful torches again. Best feature is they retain the full focus system and throw incredible well. And will out throw many an XM-L based light.
Another budget option would be to get something like this reflector and pill:
http://www.dx.com/p/52-8mm-41-5mm-aluminum-reflector-with-copper-base-for-ssc-p7-led-emitters-13803#.VRvmQfnF9yw
It’ll almost fit perfectly in a proper Mag head without mods. Only the bezel doesn’t screw down quite as far, you can file the outer edge of the reflector if you want a perfect fit though.
Grab a driver to fit the brass pill and an LED. Should be able to run an XM-L2 or XP-G2. At most you’ll just need to lift the star a little on a pedestal to get the LED in the reflector better.
Your D cells should be able to power this setup, or switch to NiMh AA’s in holders.