Yeah, you can play CGA games on that monitor of yours. Your problem is it's vertical and your machine is horizontal.
It's good to have a medium resolution monitor. They are really nice because you can play about any game on them.
NBA showtime cabs have 25" monitors. If I remember there isn't an easy way to rotate them unless you feel like ripping the whole thing out and building a new monitor mounting frame.
You can look in the project pages on the main site and find a couple of projects with rotating monitors, but man, they went to a LOT of trouble to do that. One has a round mounting board with a motor

If there is room inside the cab to actually build a new frame for the monitor in that orientation you can do it.
You can't "switch" it. It's meant to be vertical. The simplest solution is to just find another Jamma cab with a vertical mounted monitor. Keep the PCB. All the good games are vertical anyway. Like MsPac and Galaga, Dig Dug, DK, etc.
Keep the PCB in your collection. I have over 50 pcbs (and most of them actually work

I have bought and sold well over 200 games now. I have found in games the most ingenious ways of mounting both types of monitors. "Generic" cabs made by dynamo had the monitor mounted to a wood faceplate. The faceplate was square. They had 4 bolts holding the faceplate to the cab on the sides. The holes were equal distances apart. They had a bolt through them that went through the faceplate into a metal bracket.
So what you did was unbolt it and pull the monitor out and then rotate 90