Yes, you may use any of the assets/files/code that are in the project folder, it's open sourced after all!
However, you will also need to credit those who I've used their work in this fangame, especially since a lot of these assets (among other things) are translated from content originally made by Basically Games. All of which depends on the kind of sources you're pulling from the project folder.
Case in point, crediting Basically Games for originally creating everything Baldi related (voices/assets/mechanics), crediting Saintza4 and Nicky Team for providing their MIDI conversions for me to translate into GB Studio's UGE format, or crediting me for any original content not found in Baldi's Basics (like the platforming sections).