The value of the gun would never support the restoration of it. Once a gun is "refinished", the value is about 1/2 of what it would have been if original, comparable condition. A refinish would cost from $250 to $500 depending on who did it and just how pitted or rusted it is now. If the current finish is nickle plated, but a lot missing, you can polish it with various compounds, Flitz being one, and the bare metal will sort of blend into the old nickle finish. Have it inspected by a gunsmith and find out if it is safe to shoot, and use it, or as you suggested, keep as a conversation piece. Spring's and other parts are difficult to find, and only some used parts dealers may have any.