There are a zillion different LED conversions being sold commercially for the Minimag. The Opalec Newbeam is nice, giving you around 16 hours of flat regulated output from 3 leds. Higher powered Luxeon conversions are available from www.anlighten.com/shop which also sells the "MJLed", a machined down low voltage white 5mm led.

The simplest thing you can do is just enlarge the reflector hole to 1/8" (for a 3mm led) or 1/4" (for 5mm), replace the bulb with a led, and replace the batteries with Energizer Lithium L91 or Battery Station L-AA lithium AA's. The lithiums have slightly higher voltage (1.7V) than alkalines so they'll drive the LED at reasonable brightness, while having ridiculous runtime (200+ hours) and very long shelf life and cold weather performance. This is actually a reasonable plan for a cheap survival light.

Personally I wouldn't bother with Minimag conversions since there are lots of much nicer LED lights being made these days.