At a price between 10 to 20 bucks each, it's worth trying to repair these flashlights.
After removing the 4 tiny screws, it was clear the problem was the battery...
They are not easy to find but if you google for the type of battery, you will probably land in the far east. I ordered a few and had to wait a couple of months...
To repair, unsolder the 2 wires (keep track of the plus and minus!). Solder the new battery. You may have to cut 2 or 3 mm of the battery contacts.
Close the lid and put back the screws. You didn't lose them, right?
Enjoy your new flashlight!