Monitors can be difficult to repair, and insanely expensive to replace. CRTs are considered a specialty market these days, and typically only get produced for specific purposes. Thanks to how affordable flat-panel displays have gotten, and how arcades have fallen out of favor, arcade games are not considered a priority for CRT production. So getting a new replacement screen would be prohibitively expensive.
An alternative would be to get a capable conversion board on-line, and use that to output the video signal from the arcade board to a flat-panel display. This will introduce some lag, but with the right conversion board the lag can be kept to a minimum. And with the ability to use flat-panel HDMI screens, your possibilities for replacement expand dramatically. It's not an authentic solution, but it is considerably more affordable and less hassle.