If you were going to remove any other coolant hoses, then yes, draining the coolant system/radiator would be necessary first.
If the wire has been chewed off where you said, you wouldn't need a new sensor. It might be possible that you'd just need to reconnect the wires to the connector that plugs into the sensor, but you'd have to have it removed first to see what you had to work with. The other option would be to get another connector (with intact wiring at the connector) and splice that into the existing wiring harness that's going to the knock sensor area.