Try to clean them with an automotive cleaner like Scratch-X; it might take the yellowed layer off. I used that stuff on my old Schwinn Varsity and the clear part of the decals are now clear, and the white is now white.


As for the registration sticker, I think of it this way: If you try to sell the bike in the future and someone who knows their stuff looks at it and notices the stickers are missing, they may wonder if it's been damaged and repainted.