Dennis;
On the Model 20, you will have to remove the butt stock to see the serial number. It will be located on the side of the tangs. You should be able to remove the butt plate and find a serial number, too.
As to parts, watch the internet. EBay will have parts once in a while but there is always the gun sites such as Gun Broker, Auction Arms, and Guns America. You can find some parts at Wisner's too. (
http://www.wisnersinc.com) but those will be limited. Numrich Gun Parts list a new forearm at $25.70 plus shipping. You should be able to get about any part you need from then. (
http://www.e-gunparts.com) or call Customer Service at 845-679-4867.
There is no way of establishing a value on your gun sight unseen. Missing parts, condition all play a part in establishing a value, and without knowing everything about it, it just cannot be done. Sorry.........
I hope this helps.