I also had this bike headlight that my dad bought years ago and somehow, something stopped working after I used it twice. The outer shell was also very cheap and falling apart.
Shortly after the accident with the camera, I completely gutted it, not throwing anything away. I still have all the parts in a box. I was just playing around with the junk last year when I realized that the headlight lens fit perfectly in the hole for the camera lens. I got the bright idea to combine the two but didn't have a Dremel, drill, or soldering iron at school so I waited until I came home.
I played around with the idea of having the whole thing held together with a wing nut or two so I wouldn't have to bother with the teeny screws. I did not end up making that work. Also, the original plan was to keep the sliding battery door and make it so the batteries could slide in, then put the switch right below the shutter button.
I found out the battery holder was still too tall and I was unable to trim it down, so I had to get rid of the sliding door assembly and glue it shut. Also, I found a suitable screw to mount the headlight but had to spend some time grinding a corner down.
The soldering iron we have at home is pretty bad; I can sometimes hold the soldering to the iron for so long that it starts to burn me and it won't melt, but it sometimes also melts unexpectedly. I found out today that it has hot spots. I propped it up and weighed it down on the side of the table so I could be consistent with it (also because I didn't have three hands) and it worked alright. This time, the problem was that the soldering would not stick to one of the terminals on the circuit board no matter how much I cleaned it. I ended up drilling a hole through the terminal, putting a screw through it, and tightening it down on the wire. It worked!
Unfortunately, I was unable to fit the switch under the shutter button. I covered the flash bulb hole with an inner tube and stuck the switch behind that. To prevent the switch from being pushed too far in, since it was only held by hot glue, I took a piece of the old battery compartment and propped it up so it would brace itself against the back of the camera. Ideally, I would prefer to machine a part that can be held in by the screws on the side but I don't have the correct tools to do that nor do I have the patience to whittle away at something with an X-acto.
The whole process took maybe six hours, including all of the head-scratching. As of now, the two halves are held together with the original plastic hooks at the top and one of the small screws on the bottom. I might change it and use a bigger screw on the bottom so it'll be stronger and harder to lose. It bothers me that the small screws go into plastic; I've probably cross-threaded the bottom one already. I also want to put an actual camera inside the view finder hole once cameras that small become affordable for me. I have an old cell phone with a camera but I wouldn't know how to hook it up. I'm pretty sure it would need a software to run it too, at which point it would be out of my capability.
I couldn't seal this up to make it waterproof so I'm not sure it'll actually be a practical option for a headlight. It's just cool to have. Maybe I'll find another use for it indoors.