The head assembly is designed for a Surefire P60-sized drop-in. Fizz753 over at Candlepowerforums.com has compiled a very complete and up-to-date list of LED drop-ins.
I used one of my modded Cree XR-E (Q5 bin) LED drop-ins with a DealExtreme 16-Mode 3W 3.7V 7135 Circuit Board (SKU 7612) stuck in it.
Power is provided by a single protected Li-Ion RCR123A 3.7V battery. This 16340 size battery gives about 30 minutes on full with the 7612 board. I haven’t measured the current out of the board, but the specs say 1000 ma, so the output, according to Cree (Cree® XLamp® XR-E LED Data Sheet – PDF), is probably about 180-200 lm.