Thanks to Christian_Haitian (ArkOS developer) for turning me on to this fix. Turns out that the process is even easier.

In the video above (and the instructions below), I show you how to install the Linux version of the game on your Windows PC so that you can extract the assets from the GOG version of the game. Change the system to Linux, and then click on the link under “Download Game Installers”. Go into the Games section on your profile, and click the arrow at the bottom-right of your Undertale game, and select “View Downloads”. Once you have purchased the game from GOG.com, it will show up in your library. I have also been told that the Humble Store version of the game is already pre-extracted, which is even easier. You cannot extract the correct assets from the PC or Mac versions of the game, you need the Linux version. I recommend purchasing the game from GOG.com, where you can easily grab the Linux installer file, which has the exact assets you need. In order to run Undertale on your favorite retro device, you need to have the retail game assets.

Under tale download