If you're comfortable editing xorg.conf, get the machine into text mode (issuing "init 2" as root works), log back in as root then do "Xorg -configure".
An xorg.conf.new will be placed in /root. Make needed changes, test with Xorg -config /root/xorg.conf.new. When you have it, copy it to /etc/X11/xorg.conf. Then you can go back to graphical with "init 4".
If xorg.conf is present, xorg will use it. If you have a working xorg.conf for this machine/display backed up somewhere, you can just copy it over rather than generating a default and editing.
There's also a way to put just the needed changes (and not have an xorg.conf) under xorg.conf.d, but I don't know the ins and outs of that. Maybe someone can enlighten us both.