Keeppower 16650 cells will happily work.
The 2000mAh will give same runtime as two CR123, the 2500mAh a bit more.
Both will give it a bit less brightness(5% or so), undetectable to the naked eye.
Flashlightguide website has a review of the G2X and 6PX with a graph showing plots for two CR123, two LiFePo4 and 16650 cells.