Making iMac Mod - External Monitor Problem

Hi all,

I am thinking of making an iMac mod. It is a white plastic iMac (intel). The problem is, that the screen is broken. Now I could easily attach an external screen through the Mini DVI Port. But then when I start the computer the second display is not in mirror mode. I cannot set it to mirror cause the other screen is broken. Is there a way to attach the new monitor to the old internal connector or permanently make it the primary monitor (preferable even after a reinstall)??

Any ideas what I could do??

Thingsis