I hate to be mean, but from my many years of mapping, converting a decompiled Valve map into a re-skinned or revamped version is something that requires a lot of experience with hammer in the first place before evening attempting. Mainly because you are touching work that someone else has created and you are basically diving into it without knowing what you're getting into. Which can result in a really bad mess of a job and as a result, will lead to you becoming upset when people point out all of the things that made it into a mess.
And believe me, I've done ports of Valve maps to Slender Fortress, which is a horror based gamemode that I manage and it wasn't exactly an easy or simple task. Despite it looking on the surface to just be basic level edits when there is so much technical stuff to do behind the scenes. Such as cleaning up mess from the decompiler or fixing up brush work in the level that from years ago was considered fine but these days to be considered as unacceptable brushwork according to Valves own compile tools.
tl;dr, I hate to be rude or mean, but working on decompiled maps is something an experienced mapper should be attempting. As it's a learning experience to understand how to work on other peoples projects rather than "this'll be easier than making my own map!".