It really depends on what stage the satellite is in.
If the satellite still functions at a reduced capacity, then likely everything except the batteries, and fuel, is salvageable. Some of the transmitters, instruments, etc could also work, although it really depends on why the satellite failed.
If the satellite isn't functioning, then some components may be working. These components would need to be tested. The heater may not be functioning, which might make the components less likely to work after being salvaged. In such a case, the fuel, solar panels, housing, and possibly others might be salvageable, but it would be more difficult. More likely everything except the listed items would best be used as scrap material, and without manufacturing on orbit would be difficult.
Solar panels degrade at a rate of about 2-3% per year. After 20 years, they will be about half of what they were when made. There might be some use in collecting them, but overall they will have issues due to radiation, thermal, and debris degradation.
The bottom line is, the best thing is to repair working satellites, as opposed to putting two or more together. You might get some commonality for satellites of the same design, but even then, it might be difficult. I'd say the best bet is components of satellites of the same design, fuel, power generation, possibly batteries, and scrap metal is all that would really be reusable in a salvage situation for now.