A good technician starts by listening to what the generator does, then tests the system that matches the symptom. A click points to the battery and starter. A crank with no fire points to fuel. A set that runs with no power points to the alternator, the AVR or the breaker. You should hear which one failed, and why, before any part goes on.
Qualifications matter because a generator is two machines in one. The engine needs a diesel mechanic or diesel plant fitter. The alternator, AVR, controller and changeover need electrical training. Work that changes your fixed wiring needs a registered electrician who can issue a Certificate of Compliance.
A good technician also works safely and leaves a record. They isolate the set before working on it, keep every shutdown connected, and give you a written report of what they found, what they did and what to watch.