@mitu Apologies - I thought because it was on archive.org it was fair game, and the script was from further up the thread. I've removed it now.
Either way, as always grateful for any pointers from anyone, thanks in advance.
I have already tried to port this natively, there is just not enough RAM to run it...
(BTW Undertale needs OpenGL, which is only compatible with the stock Raspbian image, not the RetroPie image)
You can see my failed attempt here: https://retropie.org.uk/forum/topic/2704/undertale-for-retropie-raspberry-pi