I think the clipping made by red LEDs is very useful for electric guitars, in 
contrast to common silicone diodes which give a rather clinical sound.
This simple fuzz has two LEDs in the feedback circuit with a quite high resistance 
(high gain).
                                                                                       

The passive tone control is taken from "Ibanez Fat cat distortion". The FET gives 
the tone its final character.

The OP-amp LF351 could be changed to for instance TL071. Then, I think, you'll get 
a sharper sound. Even 741 may work.
                                                                                           

The voltage could be increased from + 9 V up to + 15 V without changing the 
character too much (even if the clipping threshold will increase).