Thanks so much for the reply! I'm somewhat familiar with how the circuit works, but I appreciate the explainer. Sadly, I've learned since my last post that won't function without the photocell, as you say. I'm really disappointed mine is missing. I'm currently working with an engineer on Fiverr to design a replacement.
Right now, the only reference we have is the photo from this thread. It's helpful, but it only shows the component side. It's been tough trying to recreate the board without seeing the underside.
If you happen to have a photo of the bottom of the board, a parts list, or both, that would be incredibly helpful. We'd really appreciate anything you're able to share!