Installing a Reeves drive, as used in snowmobiles and ATVs, between the turbine and the generator would work, although it would lose some efficiency. The principle behind the Reeves drive was invented in the early 1800s, if memory serves. A variation would use a bicycle derailleur.
None of this solves the problem of getting power when there’s no wind.
Assuming the alternator is a permanent-magnet type, and it can stand the RPM, you just switch out one or more of the stator coils, the way the alternators on British motorcycles did in the ‘50s and ‘60s.
Or change their connection pattern from series to parallel.
The original Honda 750 Fours had a live field coil, they simply manipulated the field voltage to control output. Both the field and the stator were stationary, so there was no RPM limit.