I have used Terralux and Mag LED drop ins. Either is a dramatic improvement over the incandescent bulb. Neither is really worth the money, unless you really like the old club. I still keep my two old 4D mag lights with LED drop ins, just for that reason. Great clubs, lousy flashlights, although with the LED they are better than they were new. I have a 2C Maglight with 3 lithium batteries (CR123) and a 6 cell LED drop in my truck glove box. It's a cool hack, but I'd be better off using one of those cheap Costco lights, and they would cost less than I have in that fool hack.
Today, you can go into Costco and get a pair of 2C, 500 lumen LED lights for under $20. A high end LED drop in for the Mag lights can cost $50 or more. You can probably get those two Costco lights and a collapsing baton for less than one drop in that would truly equal one of those Costco lights.
My advice would be to get either the Mag or TErralux drop ins and keep the Mag lights since you like them, and get a couple of newer LED lights, too. You can do so much better today, for so little money, that you might as well. That's what I did.
Edited to add: I followed that link to the ``900 lumen'' drop in. That will replace your reflector. It may or may not work with an older Mag light, like my two. It won't give much light at a distance.
If you want to go with a high end drop in, google ``malkoff device''. Their prices have come down, and you can get something pretty good for a bit under $50 now. If you do that, get an anti-reflection coated front lens. Here is a link:
http://www.malkoffdevices.com/drop-...mag-lite/xm-l-drop-in-module-for-3-4d-maglite