A girl at the Stockport Recorder College did this piece for her Grade 8 and a masterclass, and if I remember rightly, she took the high G flat down an octave.
As Liz suggested, a bell key fitted to your recorder or Mollenhauer modern alto will be the best option
And you're quite right, high F#, A and B flat all need knee stopping

(C# E and F on a descant)
SirP that etude is just

A pain in the neck!