If you are playing the melody rather than a harmony part you could try playing the whole piece(s) an octave up and it should sound OK. Trial & error - If it fits in OK, just play the low phrase an octave up as you suggest. If you add another piece to the length of the flute it would probably change the key from C to B? as well as giving you the lower note. I would suspect that the extension would transpose the flute from C to Bb so that it could be played along with "Fifes" as used in Orange,Hibernian or Military "Flute" Bands, or played with brass bands without the need for a separately written parts.