You could mess with R12, add in a pot there to get more volume out of IC2.1 — it's already at 1M, though; also, I've no idea if that's the "FUZZ" side of things or the transistors are what's Fuzzy.
Alternatively, you could just add a boost at the end of the whole shibang. Either:
- A 2knobjob on the FUZZ using a 4PDT with the 4th pole used to activate the boost-circuit when the fuzz is maxed out.
- An Afterblaster utility stomper-bypass with a DPDT footswitch to activate the boost instead of the dip-switches.