Wireless optical mice are basically given away free in boxes of Frosted Flakes these days. Is there really any pressing need to “fix” this one in particular via some roundabout method? Like, do you have some sentimental attachment to it, or something? I don’t think you’re likely to succeed, there. Notwithstanding that the sensors you’d need to buy would be significantly more expensive than a new mouse, but then as you’ve observed you’d also have to hack up a way to make your operating system treat your new sensors as a mouse input, and also preserve the click inputs from the separate remaining carcass of the mouse.

Really, just replace it. Any nerd should have a whole box of the things. I do; hell, I could just give you one.

The next time you paint a mouse take it apart first. The upper shell is typically quite easy to remove with just a couple of screws and then you can paint it separately from all the electronics to avoid destroying any of the functional parts.