CR123A is only 3 Volts plus high internal resistance.
It will be fine.
Different story with Li-ion 16340 of course…
Fortunately, there appears to be some space beneath the dome. Maybe it fits a simple constant current circuit, i.e. transistor/resistor.
And a small clicky would be nice 
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Yes, and you’ll gain a lot more space with a 3535 LED i stead of this big old fashioned LED.
Looks to be possible to put a 3535 LED on MCPCB plus a driver board in there
Is anybody makes driver integration inside?
I’ve only been able to place a 10mm MCPCB with a SST-20/4000K onto the copper area of that PCB. It’s off-center, but that’s not visible unter the thick dome. Nice mini-lantern.
I’d be interested in a MCPCB for 3535 and carrying LVP electronics, but there’s nothing available I guess. Still a nice emergency lantern with CR123As.