When I replaced my old INTEC black and white monitor (gosh that thing was heavy), I cut a thin piece of lexan and spray painted it to match the dash color. I mounted it over the hole left by removing the monitor using 4 small screws and nuts. It actually blended fairly well. I moved my monitor because I did not like the original location, and used the new lexan cover area to mount some other items I installed.
Once you cover the hole left by the old monitor, it will be easy to mount the new monitor on the surface of the new cover, or you can recess the monitor to be flush. Either way, you will use the monitor shell (originally designed to mount into an auto headrest}, and mount this with some type of fastener. If you are recessing it, they usually have tabs on the side that will snap into place if you cut the hole correctly.
If your original monitor is not in a good location, now is the time to relocate the monitor while you are installing a new one.
I hope this helps.
Fred