n-o-d-e (blogger) has published many mods for this watch, with videos included. They have great production quality, and the mods are easy enough to pull off. https://n-o-d-e.net/casio.html

When I did this mod, I soldered the LED on its side, so that it shone directly onto the LCD. This made the watch face much brighter.

On reflection, the similar variant of the watch with an EL display is vastly superior. The A168 is similar to the A158 you mentioned but with a better light.

You can get light spreader kits for the F91-W/A158W that replace the bit of plastic that sits behind the LCD for one that spreads the light from the LED far more evenly than the stock one.

I recently fitted one on my F91-W and it certainly makes a difference, but it's not going to make the light brighter like some of the other LED mods people have done.