How do I know if the tandem bicycle I've inherited is worth repairing? It has a serial # 459310 on it, no other identifiers
The issues you've identified aren't terribly serious--however if they aren't a comprehensive listing of the issues (such as bad bearings, etc) then they may be understating the amount of work required.

The question of whether the make and model is "worth" repairing is largely up to you to decide.  Being an unknown bike you should evaluate it on the value it has for you, rather than a broader market value.

The specific issues you've cited aren't terribly costly to resolve.  You can purchase replacement tires, handlebars, and saddles from any good used parts shop for a fairly low price.  A new chain is a small investment, and is a regular component to replace.

Overall though I presume your question is omitting some details.  Photos and a more comprehensive listing of known issues will help better evaluate the potential to return the bike to a functional state.