I am presuming that is a O/U double bbl? If so a few of the parts are available at this address. Scroll to the bottom of the page and you will see two model 90s listed don't know whether 12ga will fit but I don't really know what parts you need for sure. There is a few parts schematic but they have a full parts schematic available for sale. Next to the last part listed under the 90ST is a 20ga forend.
Numrich Gun Parts Corp. - The World's Largest Supplier of Firearms Parts and Accessories