I just acquired a Mystic Color Classic that had a display that was entirely red. At least the green gun was not working, and perhaps the blue too (I couldn't really tell). After pulling, cleaning the contacts on, and reseating the motherboard, the problem cleared up and full, accurate color restored to the screen. It's quite a simple fix, but it might be something you've overlooked, thinking it was a monitor/analog board problem.

Also, having never attempted the Mystic upgrade myself or owning an already upgraded unit until now, is it normal for the screen to be squashed with a large-ish >1" border on the left and right edges of the screen? I haven't taken the time to open the machine up and look for adjustment pots like are on the B&W compacts.