You are not legally allowed to make a Friday Night Funkin' copy with the same name since it's trademarked.
As for the mod part, just basic ethics
I’m not talking about a copy with the same name i’m talking about the engines + similar games people make and about the unofficial fnf mobile games on playstore, and people make ports of mods for different game engines and that is also allowed. hope you understand that i’m not stealing Mr. And i never will :D.