I posted on this last night, but my post has vanished
What are you looking to get from the mod? Throw? Flood? Extreme brightness? long runtime? What battery types are you going to use? How involved are you willing to get?
The easiest mod would be to replace the nichia LED's with GS models. Direct drive with the LED's in parallel and the appropriate resistor could give you 30 lumens with no heat issues, and long runtimes (though with diminishing output due to the resistor). Alternatively you could use a more powerful LED and a 1x7135 driver board such as
this, though you'd need to find a source for less than 20. Running at 350ma should give minimal heat problems, be gentle on the batteries and also give about 100 lumens. You'd need a custom optic and some sort of heatsinking solution, so it's more involved than replacing the emitters for GS versions, but brighter and you can choose whether you want more flood or more throw with your optic.