Can someone help me understand FE better? Not really sure how to go about fixing this gui script. [closed]

So I'm trying to fix a game that broke from FE from my understanding I don't actually own the game it was originally created by Drager980, but he ended no fixing the game for whatever reason and making it open source so me and many other fans of the game are pretty sad about this.

Anyways I wanna try my best to fix this game and everything seems to be working fine besides all the GUIS and I think this is where remote events and remote functions come in now I'm not very smart, but I understand that it's like one a client makes a chance it needs to go to the server or something like that anyways I just wanna know how would I got about making these gui scripts fe compatible without having to rewrite everything.

A quick little summary of what the GUIs do: So in the game when a player joins a GUI pops up (which works) and they have all these buttons like Owned characters, store, upgrade, etc, but the issue is you can't click on anything and none of the characters pop up and this is the only issue with the game which makes it unplayable.

So again how would I go about fixing this kind of an issue because I've never actually had to do this before and I don't fully understand fe if that's even the issue. I can share the scripts or screenshots if you don't understand my issue.