I think it will be really hard to get the existing pan out of there without compromising the walls. If starting over, all options are open.
I'm having a hard time thinking of a way to patch this and have it work without replacing it. Maybe someone else will. Good luck.
Given the dimensions, you should have about 5/8" drop from the outside edge to the middle where the drain presumably is. The pan may not be that thick, but if you used a router with a circle cutter and had the router on about a 2-degree angle, and kept making circular cuts further and further out at that slope, you might get it sloped, and a new surface. I'd want to try that on something else before I tried it on the pan. Kind of far out choice.
But, if you can't get to the drain to lower it, that would be useless. I can't think of anything you could use to overlay the existing pan that would work properly.
You could probably use an epoxy thinset and tile it, but that would just follow the existing slope, which you implied doesn't exist, and then, you'd need to play with the drain to raise the grate. There are some ways to do that, though. You'd be able to get a very slight slope with the thinset, but by its nature, it's not designed to be very thick. You'd want to talk to the thinset manufacturer to verify it would bond to the solid surface material.