It is relatively easy to take a digital picture of a decal and "fix
it up" with PhotoShop or any other image software.  You can then print
it on transparent film and voila, a perfect decal.  If you are doing
detail on a glass plate the same thing works.  Simply put the film on
the inside of a glass when it is ready.  (Print it in mirror image to
do this.  The software allows for that.)  You can brighten up, copy and
paste to replace missing parts, pretty much whatever it needs.  It can
be intricate and detailed work, but it yields great results.

Julian M. Brook