A lot of information about how brushes, entities, textures and other data is lost when a map compiles. For example hint and skip brushes (used to influence the visible information set) are removed after compiling so you will need to manually recreate them. Other tool textures are removed (including null which in some cases can break the brush).
Some entities such as texlights are removed too I believe but I'm not sure, and all objects are no longer 3 dimensional but converted and subdivided into series of planes. When decompiling it tries its best to give you something that will represent the map as best as possible but in no way will be close to the original in any shape and form
There is nothing you can do except ask the author for the map file or find and fix any errors or major differences you find yourself
Unlike source maps goldsrc bsps do not contain the original brushes, but you can try Winbspc. It does a decent job of recreating the brushes from the faces, how much fixing it will need depends on how complex the map is.