Im not sure a copy of my class will work as i think there are Cast to RPGUIKit_FirstPersonCharacter nodes in some places which is casting to a specific class. A child class might work though and I think is probably better because when I update the package it will be less of a pain for you to integrate the new version into your existing projects without redoing a lot of work.
I also noticed you omitted which player controller your project is using. Please supply a screenshot of your Maps and Modes area in Project Settings and the Game Mode area of your World Settings (which can override the Maps and Modes settings).