This is an interesting mod.
Finding the Muff between fuzz and distortion,
this mod make him sound more like a distortion.
It makes him scooped and more bassy, afterwards
you may need a screaming bird…

http://www.kitrae.net/music/big_muff_guts.html
Its about the limiting resistors R12 and R19.
I used 22k parallel tho stock 10K.
And some 6,5K parallel to 2,2K emiter resistor R4 in Q4.