Do you have a dedicated or a conversion from a world or USA?
The USA and world games used a 25 inch U5000 or K7500.
Exotica used a 27 inch. The games are medium res so you can't just use any monitor you find.
You are probably going to be stuck using a converter box and a LCD screen. I think a 29 or 27 inch LCD will fit in the cab fairly well but of course look kind of bad with the horizontal stretching.
If you want to go with a monitor a makvision 27 inch is pretty much your only option for new tubes.
http://www.xgaming.com/store/arcade-parts-and-accessories/product/weiya-29-flat-screen-crt-arcade-monitor/