The most important differences between models are the high-pass capacitor values, the feedback capacitor values, and the RC values of the tone control.
This is EXACTLY the case. You can also make sight changes to the gain of the distortion stages (make them more sensitive) by increasing the collector to supply resistors, or reducing the emitter resistors. However, the changes will be marginal.
I find it astonishing that some idiot musos will pay literally thousands of dollars for a cheap and simple effect like this. I saw one on Ebay a while ago that went for nearly $4000

because it had been used on some album or other.....
I remember getting my very first BMP in 1970, and taking it apart. I was disappointed to discover that it was just four cheap transistors, four switching diodes, three pots and a handful of resistors and capacitors. It was just a week later that I sold my first - improved - clone of the BMP to a friend in the neighbourhood. :tu: He still uses it today, so I must have made a good job of the construction! It was what got me into building guitar effects.
The only thing wrong with the stock circuit is the dreaded hiss - the gain of the distortion stages ensures that this will be ever-present! I've built them with noise gates, and even with envelope followers to retain the original guitar envelope (a truly bizarre effect, with a fuzzy, distorted, intermodulated mess that comes out of the Muff, but with the original attack, sustain and decay of an untreated guitar!). You can tame it a bit with a low pass filter, but you'll change the basic tonal character.....
It's really worth building one yourself. Throw it together on Veroboard. Make sure that you keep the input well away (physically) from the output to prevent building an oscillator! Try different coupling capacitors and caps in series with the diodes, until you get just the sound you want. Even if you had to buy all the parts, it shouldn't cost you more than a few £, $ or whatever.....
Just don't be fooled into paying massive amounts for something that anyone with even the most basic constructional skills can make in an hour or two!