Yes a compressor can be changed out, system evacuated, and recharged on a roof AC just as it is on window AC's. Service valves can be brazed on. Though I have replaced compressors on window AC's which are basically the same as a roof AC (only difference is the cabinet) the cost is not worth it.
On your AC it could be just the start controls (relay and capacitor) are shot. And that would the cause of your 30 amp CB tripping. It isn't doing it on your 50 amp hookup because the thermal overload of the compressor is shutting it down first.
You could invest in new start controls or try a hard start kit, though I don't like using hard start kits. The other option is a new AC.
