Well. I think it's ok if you make it for yourself and don't redistribute it before you get a permission.
So how to do it:
For textures(materials) : (hi-res vtf files) Copy and paste job. Use the VTFEdit if necessary.http://bsnooch.com/forums/index.php?topic=530.0
(vmt files) Use Fakefactory's version. However, some material proxies may not be available in source 2006. These material errors will list in the console whenever they're loaded (for easy debugging). Remove some lines in vmt files if necessary.
Beware, don't expect the textures look exactly the same as they do in orangebox engine.
For custom models : You'll probably have to decompile them and compile them again for source 2006 engine.
Anyway, it won't be easy to fix everything. Good news is you don't need everything working in perfect status before you can have a visible improve in the graphs. Just copy what you like and fix these up.