It can be restored, but it will likley decrease the value rather than increase it.
If it were mine, I would disassemble it, scrub all the metal parts with solvent then coat with Tetra Gun oil. It's a synthetic oil that coats the metal and does not varnish up.
The stock can be cleaned with Scott's Liquid Gold. Soak a rag with it and scrub it pretty well to remove the grey/black crud. Two cleanings should take care of it. If it needs more, just wipe it down once a week with a rag soaked in Liquid Gold. Don't store the rag after you're done, throw it away.
If you are tired of the rifle and looking to sell it, send me an e-mail.