What kind of iMac do you have? If the only thing that is broken is the display and everything else works, it seems that all you need to do is send the video feed to an external monitor. How you connect the monitor depends on which iMac you have.
Trying to take apart the iMac and put it into another computer case seems much more work than simply hooking it up to a second monitor. Of course, you might have problems if the computer doesn't put your desktop on the second monitor and you can't access the system preferences. Good luck
Of course, you might have problems if the computer doesn't put your desktop on the second monitor and you can't access the system preferences. Good luck