Savage acquired Fox in 1930 and kept making the Sterlingworth until 1942. Serial numbers ran 50,000 to 200,000 for 12 ga, so yours is fairly late in that period.
Value depends strongly on condition; $275 to $1250 in the Blue Book, but less 25% for Savage production. Advertised prices in the Gun List seem to run on the high side, though.
Numrich has a lot of repro buttplates.
www.e-gunparts.com
You might could get extra barrels from Connecticut Manufacturing Co. who are making new Fox shotguns. They won't be cheap, their new manufactured CE grade (they don't make anything as plain as a Sterlingworth) is a $7200 gun!