So I downloaded basically the whole SVN "thing" (yes I don't know much, and I need to read more. What a joy) a gig and a half in all.
From what I can understand it needs to be (eventually) compiled although it plays uncompiled.
There is a tremulous branch although I am not sure how to get it to work and I'm trying to understand the structure of XReal. I do not know where it would see mods, it could be as as easy as putting the trem patch/mod in the correct place since it RUNS with XReal and is made to work. My friend knows more than I do but both of us don't know much regarding development., but with enough reading anything is possible.
If we can get the mod to work and everything checks out then we can redo a map(materials, etc),
high res textures with proper licensing (that will be easy, NOT) and finally see what happens then utilize as much of XReal's features while making everything looking good.
From what I can see 2 maps are playable with XReal, if I get the branch to work
(I'm emailing a dev in a minute, hopefully I'm not wasting his time) then we have 2 maps to see how they work for XReal and why. dissecting something that works is always better than nothing)
Remaking assets(Aliens humans etc) is something you shouldn't be able to mess up and is rather straight forward,
especially since it's MD5's

But being straight forward doesn't mean it won't take time.