The minimum needed by law will be dictated by the aircraft 'top drawing', which in the case of the Jet Ranger is dictated by a very old regulation (CAR 6), but which is pretty well mirrored by Part 27. Flight instruments needed are pretty minimal from this regulation - airspeed, altimeter, turn and slip, wet compass, and not much more.
Part 91 doesn't add much.
For commercial operations, check Part 135. If memory still works, this requires an attitude indicator and gyro compass for night flying.
But the list are interesting for what they don't require!