I actually DID manage to locate somewhere around 100-200 previously unavailable original marquees.
The rest (the fakes).
Are either
Modified title shots (extremely easy to spot these).
Scratchbuilt using graphics from flyers, sideart, console versions, or whatever else I could find to use. These range from easy to spot to almost impossible to spot. Some I spent seconds on, others I put quite a bit of time on. A few I was able to find an unusable marquee image (too small, and angled, etc), and was able to mostly reproduce the darn thing.
Then there are a few that are simply the plain title in the Atari font (easy to spot).
Finally, one game (Qwak, or Quak, or whatever it is), has a marquee from a different game with that same name.