I defined them based on what I could see while leaving some of the bleeding. I know some people will object but that's what I felt was the best representation of how it's supposed to look.
So this is the scan:
And this is the current artwork:
I think the best thing would be to just clean all dirt and hairs from the scans (which is what we had been doing until Pinball, I think?) and try to avoid any kind of artistic interpretations, for the sake of MAME's emulation being as historically accurate as possible (and leave changes/vectorizations/fan-made backgrounds to HBMAME, maybe).
If something happens to look a bit blurry in it, I would personally prefer it to stay that way until a better scan surfaces, but of course that's just my opinion.
And yes, I know that most of the altered elements in this case are hidden anyway, but I think that people should be able to have a look at the original assets for research purposes, just like they can check out the original graphics/sounds/etc inside the ROMs with the appropriate tools.