Was found in garbage, missing one key, and no power cable. Is it worth trying to see if it works?
Depends on the version. The MK1 needs a PSU, the MK2 S88 needs a PSU. Other models can be USB powered. Take a look on this page and see what version you have - https://www.native-instruments.com/en/products/komplete/keyboards/kontrol-s49-s61-s88/downloads/
You can also download the software from there. If you have a unit that needs a PSU try searching "native instruments power supply" on Amazon and you'll almost certainly find one.
Depends what version - is it a MK1/2/3 ? You could just plug it in and see what happens?
Do you know where I could get a replacement power cable?
Yhank you!