An old enough dragon can learn spells that grant flight with improved maneuverability. There are the Wings of Air and Wings of Air, Greater spells (cannot link as they are not OGL). That can get a dragon from Clumsy maneuverability to Average. But, a good old-fashioned Fly spell can give them a 60ft fly speed with Good maneuverability.
Fly speeds to do stack. But how do maneuverabilities work? If a dragon casts Fly on themselves, can they move with good maneuverability so long as they only move 60 ft in the round, but if they move their full 250, they have whatever maneuverability is associated with that speed (probably clumsy)? Or, do they wind up with 250 ft fly speed with Good maneuverability for the duration of the Fly spell? Or does the Fly spell make their wings inoperable, and they have 60 ft fly speed with Good maneuverability only?