Well, you can switch the diodes out or do an asymmetrical clipping scheme. You could also replace the transistors- I feel like I may have heard something about BC109s, but maybe I'm wrong. Those seem too low-gain to replace 2N5088s. 2N5089s are a lower-noise version of 2N5088s, you could try those. Input and output cap changes would probably work well. The Suppa Tone Bender is a very similar circuit to the Muff, but with antiparallel diodes going to ground in the tone stack stage for a different sort of clipping. You could try doing that if you're feeling funky, I suppose. Having a TB/BMP switch would be pretty sweet.