Lens focus plane is slanted - Canon FL 58mm f/1.2

I have a Canon FL 58mm f/1.2 that has its focus plane slanted. What would the typical cause of that be? It is visible even when stopping down to f/4 with an infinity focus. The left third of the picture will be in focus while the right third will not be.

What's the likelihood that a repair amateur could get in there with a lens spanner and reseat a few glass pieces? My best guess is that one of them got knocked slightly out of place. Nothing looks visibly wrong with the lens at first glance. I tried it on 2 different cameras and it happens with both.

I found the issue, highlighted in red. The gap between the stationary and rotating part of the lens is slightly different on the left and right sides. There is a small amount of play. I can force it back to an even plane by pressing down on the left side. When I let go, it's uneven again.

That would do it. It's probably just a set-screw (or two) that has come loose. Can you fix it? Sure, assuming you can get at the problem screw(s). That may or may not be easy, and you have to have the right tools.

Thanks. I found a disassembly video on it but it's out of my league. Can anyone recommend a good lens repairer in the USA?