You need a good baseline image, if you do not have one without flaws, fading, veining, or cracks.
Use an existing backglass scan (for those machines that have already been done as there are quite a few), or ask a fellow collector who owns a particular glass of your game as a donor for a full scan. Today, you can make a pretty good one run replacement using plastic film and cutout the score areas, and then back it with another pure white piece of film for proper transparency.
Keeps the cost under $100.
What does this NOT do?
Provide a replacement when a backglass has specialized mirroring, cutout areas, or specialized effects that were imbedded with the silk screened ink process.
A good example is Haunted House (Gottlieb, 1982) with the lightning strikes, because it wont work at all.