If the motherboard of your iMac is still functional, you can get the AirServer software, It is only $14 buck to buy, from there you can either mirror(or extend) your other Mac to the old iMac display with no mod, no wire or cable required as long as both Macs are on the same network. For $14 the AirServer is well worth buying.
"Maybe relevant - I carefully followed the steps here [1] to successfully convert my old desklamp iMac 20"" to a DVI monitor. I hooked it up to my desktop Linux machine (of course hidden under the desk :) and felt as happy as a man can ever be. So great to give this old Mac a new leash of life! I think the principle is same for this new model too.
How to do the same thing, but using a 17” Macbook Pro’s screen. The process is more or less the same on a laptop as it is on an iMac, but the iMac has the luxurious added benefit of there being so much space to work with.
