If you turn the gain down on the muff and use an graphic EQ in conjunction with the muff’s controls you might be able to get some approximation of a fuzzface sound but it’s not really something that’s easily possible, they are totally different circuits and very different in sound, so it’s not going to be a case of changing a few components I’m afraid.

Gut it and put a Fuzz Face circuit in it.

Really, if you want a Fuzz Face sound you should buy or build a Fuzz Face. The Big Muff’s a whole different animal.

Yep, not gonna happen no matter what you do to it!