hi christo,
i dont know of any physical way to remove fading or yellowing from a negative. nor scratches. dust and mold are different and can often be removed with a very soft dusting type brush. but be careful with mold. dust and dirt and stains are usually sitting on the surface of the negative where mold is sometimes something that has eaten into the surface and you want to be very careful about handling that because you'd be brushing away the film as you brushed away the mold.
but the first thing is always a scan or a reproduction through photography. get a capture of the image even if it's dirty as sin. then do what you can physically to clean it up, if desired, and then do another scan for cleaning up digitally. this way just in case your physical cleanup goes badly, you've still got the original scan to work with.
you might also check out the history forum for threads on physical cleaning and i started one here in the restoration forum as well. see
HERE