My experience with scanning existing sheets and trying to print decals from the scan is mixed.
At the very least, the scan will need to be cleaned up in Photoshop to remove the yellowed clear
around the image. Sometimes, it takes a LOT more work to clean things up to the point that the scan is at least usable.
When you are dealing with an old sheet that is difficult to replace, I always recommend that you get a decal maker
to take the scan and redraw it, usually in a vector program like Illustrator or Corel Draw. The printed decals will be
clearly superior to just printing the scan. The bonus is that the colors can be corrected or changed easily, and you can change size also.
If you are making a scan of a decal sheet, save the file as a .TIF file. This ends up being a bigger file, but the image is
not compressed, so you get the maximum image quality.