Yep, standard CGA monitor. Any classic monitor will work. You have the original K4900 in there that was shipped with the game (all I've ever seen in Qberts)
Also note that there is *very little clearance* in the cabinet between the neckboard on the picturetube and the back door with the PCB mounted to it.
In fact, I find it very interesting/troubling that when they converted the cab they decided to put the tiny Arkanoid board on the back door rather than simply mounting it to the side below the power supply. They could have used less wire and this would prevent any problems with the board getting too close to the monitor/etc..
In any case, if you do put a new monitor in, BE SURE YOU KEEP the current "vertical" shelf mount monitor frame that the K4900 is in. It's very rare to find these mounts and you can't buy a new monitor with them. If you buy a new monitor you will have to move everything over from the new frame into this existing frame (tube, chassis), because even if you were to build a mount to hold the new frame, it would likely stick out the back of the cabinet, depending on exactly how it is designed, or at the very least hit your existing PCB on the back.