Of course you can "pirate" Undertale, but you will not be able to install the Linux plugin to it and I don't think it worth it to play 1.001 on Windows. So, you need to buy steam version or do runs on free version but only on Windows system.
That's simple. You just need program called AutoSizer. With this program you can set your own window size. For calibrating you need open the program and open the game, then you need to find Undertale in "Currently open windows" in AutoSizer and you just click on "UNDERTALE" (get sure it is the game, not the folder) and chose "Resize / Position" and write any 4:3 resolution, for example I use 1280x960, then press "Center on screen" (otherwise Undertale will being opening somewhere in down or left side and it's very important if you need to open the game multiple times while speedrun) then press "Set size", press enter and done! After that you can drag LiveSplit window where you want and start running. (Very huge apologize for shitty english) P.S. LiveSplit is never visible in fullscreen mode, you able to play only windowed
Create a new folder and apply firstly Linux on it and then WPM