OK so what you're saying is the monitor is already wired for mirror use.
Several factors come into play, how the monitor is mounted, whether the game had dipswitch settings for flip, whether it was hardwire flipped, whether the monitor had been replaced at some point in time, blah, blah, blah.
A few possible combinations of all the above.
Like grantspain said, just set it up and try it and then go from there.
Most likely you will only have to flip one direction, horizontally. (i.e. the picture will be backwards, but not upside down)
It'll be easy enough to do once we know what you are looking at after hooking it up.