My bathroom shower handle needs replacement -- it started falling off recently. It's damaged and needs replacement.

What kind of a handle would work?

Below is how the rest of it looks like. Note that I don't know the brand, it's not written anywhere. Nor do I know which year it was produced.

My bathroom shower handle needs replacement -- it started falling off recently. It's damaged and needs replacement.

What kind of a handle would work?

Below is how the rest of it looks like. Note that I don't know the brand, it's not written anywhere. Nor do I know which year it was produced.

Edit: here are pictures of the handle itself:

Note that the outer hex bolt is stuck (sitting for hours in WD-40, no success turning it after that). I might be wrong, but I think there are two inner bolts on either side of the inner square piece. One is stuck, the other is missing entirely.

I tried multiple handles from the hardware store and none of them really worked.