to get 1.001 you need to go to steam, find Undertale, click properties, Click Betas, you want to select "old_version_101 - old_version_101", re-download the game or something Boom you have 1.001. Also something you should do (because steam loading times are slow) is disconnect steam from Undertale. The way to do this is actually surprisingly simple. You want to go to Undertale's local files and copy all the files somewhere you will remember (like your desktop or something) then you want to open that folder you just moved from steam and then look for something thats named like steam_api.dll. You want to go delete that. Boom you have the most optimal version of 1.001 (windows)
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.