While a very good hunting/ plinking quality rifle, it does not have a high collector's value. In good condition, it is about a $100-$125 rifle. FWIW, if you register on Gunbroker, you can search COMPLETED auctions, and see what a given gun has sold for in the past- to me, best indication of value.
Want a magazine? http://www.midwayusa.com/Product/113294/marlin-magazine-marlin-20-25-80-780-22-long-rifle-7-round-steel-blue
Stocks can usually be rehabbed with some Formby's Furniture Refinisher (be careful- stuff is flammable as all get out) and some oil finish (Tung Oil, Tru-Oil, etc)
Unless metal is horribly rusted, minor rust can be improved (not fixed) with light oil soak, and a rubdown with a COPPER Chore Boy pot scrubber, followed by rubdown with clean soft cloth.
IF rifle is mechanically sound, would get a mag, and clean it up. Would not be that expensive. They are good shooters- and sentimentally, priceless.