Today I received a headlamp; a cheapy similar to the D25 that has been reviewed a few times here. Mine is the single LED version, also available under some other names. Mine was from an Aliexpress seller.
I bought it without a cell as I have plenty of known quality ones already.
I have already done the simplest mod. I replaced the reflector with an optic I had. The optic was one of those 20mm diameter types that come with a white or black plastic holder. I removed the holder and after filing down the diameter a small amount, the optic fits very well. When the lens cap/ring is screwed back on with the stock glass lens in place it is a nice tight fit. The optic is a 60-degree type IIRC. The beam is nicer, for my purposes than the original with the reflector that has a pronounced hot spot.
The UI is actually not bad at all. I may not touch the driver, but I may swap the emitter for something a little warmer. Have to look through the parts on hand.
Hard to beat for about $10.