So, would it be okay if I kind of cannibalize this project for use in a different one? I assume so but I was just wondering, of course I mean the code, obviously Undertale specific stuff is owned by toby , and your art is yours. So yeah. Also, any idea why the menu's disappear when I played it in Gamemaker Studio 2? Also also, I originally glossed over this because I mistook your user name to mean that this was somehow made with python, silly me. Nevermind the first part, I guess I should have read the description more closely.
Of course you can use the project, that's the purpose of its existence, to give people a head start building a system.
In regards to the menu thing, with no supporting information, it would be impossible for me to tell you why the menu disappeared. This was written in GM:S a few years back so there's no import support.