That's not the point. You're imagining a ridiculously disastrous scenario where every FZ in the world gets this problem. And it's not such a huge disaster you have on your hands either, because with the two examples you posted you can still edit. The parameters you can't see can be done by ear. You can still navigate to the parameter perfectly well! OK you were unlucky with your machines, but that's all.
Notwithstanding that, I'll hazard a guess at a replacement display solution. I don't know much about displays, and logic circuits/ CPU output was never my strong point. I don't think an Arduino is needed, the programming is already "there" so to speak. I don't understand what the SD card is for? The hard part is finding a display of this size. Maybe it's possible to run two or three standard oleds with some kind of voltage conversion for the levels? The conversion part seems straightforward, hideawaystudio has done it for several of his synths. All you need to know is Ohm's law and how to read a data sheet

More pragmatically, you can use the old FZ editor on an old PC with FZF format, convert in the hxc software and maybe tweak later on the Casio. That's what I do with mine. Easier than menu diving all the time. As a bonus it has a really bad sample rate converter, sounds nice and crunchy
