@grammargirl If I remember right, I believe @anildash actually modded his for this purpose.
I otherwise see a ton of good suggestions in this thread that beat me to the punch. Luna display will probably be easiest, though you could always do something like Duet Display if you don’t want to buy hardware.
My suggestion would actually be to look at getting an HDMI capture card. People are doing this to plug in Switches on iPads and I imagine it would work with your old Mac, too.