Light will run on 4.2v if the cells are parallel (can you run it with only one cell?) Or 8.4v if they’re on series…. I would assume the 12v would be too much, plus, if its anything like a car engine, you’ll be getting voltages of 14v…
I don’t think that headlamp is that good, but if you like it that’s what counts.
Try this thread if you still need 18650 cells.