Original CS: Source maps, models, skins and so forth are stored in the Steam cache files, so you can never delete them.
Therefore, things like Fusion Pack just place the models, skins and sounds in the exterior CS: Source folder, not in the cache file. So, if you remove the models in the CS: Source folder, Steam uses the content from the cache file again.
You should be able to just delete the models, sounds, etc... directories that were created by Fusion Pack, which brings back the old models.
If you lost a custom item however, like a weapon model that you installed, you will have to re-download it.