but there is a solution to connect the original screen to a mini
CLICK - but be careful - if you don't understand chinese - it will be very hard to get ;-) (google could help - but not so much)
there is a topic about that on macrumors, but last point was the link to the page above.
i have an iLamp 17" and i'm also planning a "technical refresh"
but instead of a mini i will put in a mini itx board with a core 2 duo - the mini wont help anyway - the optical drive is not usable in the iMac, the whole thing makes only sense when put in without the case and in the end - the mini is only a mini itx board - so why should i buy a pretty expensive mini itx board without the abilitys that some other boards have (mine has 2 vga and a hdmi output, 6 usb 2.0 and since it's a santa rosa board - i'm able to run the snow kitty on it
i'm thinking abit getting a different lcd panel in the g4 - the hanns g. 17" have the same aspect ration and the same resolution as the 17" iMac g4 - but i have to figure out how to put everything together in the lampie