It's going to be very tedious, I'd advise to not bother. If you change the resolution of a texture, you'll have to change so many other things in the BSP as well, i.e. scale down the UVs, which leads to more world polygons (they're subdivided every 240x240 texture pixels), and you never know if you're gonna run into lightmap limitations.
There is no such thing as an "accurate decompilation" because the BSP simply doesn't have much data. It has processed faces, but not whole brushes, best decompilers can do is guess. This means no clip brushes, sometimes no triggers either. If a face is optimised away and removed by the compiler, you cannot get it back. There's just not enough data in the .bsp, simple as that.