You can assign a character to a different key in Windows. It gets tedious to assign a lot of characters to different keys, but can be done. It's simpler if you assign to an Alt+key than making just a single key print something different. You can asign whole sentences or paragraphs to a single Alt+key fairly easily.

You can also choose from more than a hundred different keyboard layouts by setting to one of the keyboard layouts that's standard for a different language. Purchase of the "foreign keyboards" is generally only easy if you live in (or go to) a place where the language is fairly common; but probably could be arranged elsewhere.

On most current keyboards, the "key caps" can (at least in principle) be removed and replaced and most of them are interchangeable as long as they're the same size as the one they replace. Replacing one that falls off with a new cap works better than just moving them, since prying them off and putting them back on sometime "loosens the grip." (When a keycap falls off, you can usually snap it back on if it's not obviously broken; but you should expect it to come off again fairly soon.)

You could paint them with Whiteout or an acrylic paint pen and (when it's fully dry) write your own characters on them with a "sharpie."

(Ugly is fine, when it's easier and it works.)

John