I believe the original monitor it shipped with was a WG K7500. permitting it was never replaced, of course.
the one in our Carnevil actually has a Samsung tube instead of a Zenith one, probably explains why it's held up better than most of the other Zenith crap WG was using back then. of course, if it's a K7500, your results may vary.
one thing I'd try first is finding the remote adjustment board. it's probably hanging somewhere in the coinbox (also assuming you're talking about a dedicated Carnevil) ... once you find that, try turning the contrast up first. you don't want the image to bleed at all, if you see green rays coming off portions of game graphics (like the text) then you need to turn it back down until the bleeding goes away.
then work on the brightness. only turn it just high enough to where the black portions of the screen don't show up. you don't want the black image to have a greyish or brownish tint to it.
if neither of these improve the overall vibrance of the picture, the next step is to go to the flyback on the monitor chassis and adjust the Screen pot on it. this is like the master brightness. turn this up very gradually, have someone be your 2nd pair of eyes or have a mirror to see how bright it looks. it doesn't take much to make this increase. if you see diagonal retrace lines in the picture, you went too far.
this applies to any kind of monitor. if you're not satisfied with the results, then it's time to find someone nearby with a rejuvenator to test your tube.
alternatively, another cause for inaccurate guns in this game is the +5 voltage. I felt like a total idiot when I metered ours and saw it was running at around 4.9V. you want about 5.10 volts at the JAMMA edge, maybe even 5.15. if you have questions on how to do any of that, PM me, or post in the General Repair section and someone can help you.