Create game in UDK 2015

I’m trying to create a completely free game, based on UDK 2015
I know it’s old, but that’s what I managed to use on my PC; however, I have some doubts
The first, obviously, is whether I can still create a game, based on UDK 2015.
And the second is if I can use as a base, some classes (scripts) from UT3 and UT2004, to create my own classes in the UDK, making adaptations to the UDK language, and also making the necessary references to Epic Games.
It’s not hard to find the source code of these games on the internet; but I still can’t understand what is the purpose of publishing this? And how far can I use these classes.
In this case, my intention is not to use the elements themselves, from these games in mine, but to adapt unofficial content (mods, mutators, etc.), created by other people, for these games, and making appropriate references to them also.
I was studying these scripts, and I realized that yes, there is the possibility of using this feature in the UDK; although there are so many changes to be made, that I would end up creating new classes, instead of just copying what has already been done, which I don’t want to do.
Furthermore, I also noticed that some UDK classes are pretty much the same as UT3.