Consider socketing the 1K5 resistor to experiment with values to get the best sound out of it, or maybe replace it with a 10K trimmer.
Description from Music Radar:
"The daddy of all stompboxes: the first commercially produced transistor-based guitar effect pedal and also the first fuzz box. First made in 1962, it took until 1965 to catch on when Keef used one on Satisfaction – and the world of guitar tone was changed forever. The FZ-1 used three germanium transistors to amplify and clip the guitar’s signal creating fuzz, glorious fuzz…"