CR123A batteries run at 3V and perhaps 1400mAh. Two of them will double those figures. An 18650 runs at 3.7V and as much as 3400mAh.

I don't know enough about your modified strobe to estimate how it would operate at a little over half the voltage. I'm guessing that if it's not designed for different voltages, you're best off sticking with CR123A primaries.