Dorcy 41-1644 40 Lumen 4.5V - 6V LED Replacement Bulb. $4 at Frys or Sears. $7 at Dorcy Direct if your local stores don't have it.
https://www.candlepowerforums.com/threads/288283
Not as bright as the stock 4D bulb but your batteries will probably last until they leaked (I estimate ~50 hours runtime).
About the same brightness as the Terralux TLE-1F but a lot cheaper.
TerraLUX TLE-6EX
Roughly 2X the brightness of a stock 4D Mag.
Sino Union Fusion 36
Nasty has already posted the link.
About 8X the brightness of the stock 4D Mag. Note more of a big blob of light than the spot/spill of most current torches.
TerraLUX TLE-300M-EX
A more focused beam than the above.
Malkoff Drop-In Module for 3-6 D-Cell to fit Mag-Lite
You need to cut off the cam from your reflector or buy a reflector with the cam precut.
-
EDIT
Maybe just get him a new light.
That little penlight called the Preon 2 WARMS (top in photo) just blows my Mag away. $43.
Mag 2C left, Preon on high right.