Hi

Normally it's only the polarizer which needs to be changed, not the screen (if you have only fading sprites). If you are sure it's the screen itself, then you have to change it in one piece, because it's a liquid display screen and can not be really fixed as far as I know, so you will need a spare screen (see on ebay, or spare whole faulty games with a good screen for sure )

and here some advice how to test:
If you are not sure if the screen or polarizer is the problem, then you can try it as you can see in the pictures below

1) shows you a faded screen on DK II (upper screen)
2) shows you a polarizer bought on ebay for some bugs, you can test before fixing
3) shows you an option if you don't have any polarizer, take 3D glasses, one side is perfect for testing the G&W screen









hope this helps, cheers, gameandwatch

Polarizers are easy to replace. You can buy replacements on ebay for cheap.
I did it for my Mario Bros when I got mine. One of the sides looked pretty damaged by sunlight or something, but I couldn't tell from the ebay picture. I let it slide and just decided to replace the filter. Looks brilliant now.