Before you do anything.. Be aware that "restoring" or any sort of refinishing can knock the value down of an old firearm. If you are interested in retaining that shotgun for collector value, you may want to have it appraised or inspected.
Original finish and condition are desired over any restoration when value and collector status is concerned.
If none of that is of concern and its just for your personal interests and usage - good luck with your project. Theres a strong chance that some other company actually made that firearm and Riverside put their name on it. ...sorta like Sear, JC Penney, Western Auto, etc shotguns and rifles. We need to figure out if that is an actual Riverside production or an off brand of another company. With that having Browning on the barrel, that'd be a good place to start looking for matching Browning models. Once you match one up, we can then look for parts.
Parts: Numrich's -
http://www.e-gunparts.com/
As for bluing - hopefully someone else can chime in. ..no experience here.