Cool! Thanks for the review. Feel free to build your own to play around with! I designed it so that it would be easy to swap out key components to get different tone out of it. Changing the caps that feed the Timbre control are definitely a good thing to tweak. Going slightly higher with the input cap can probably give that extra "oomph" for people who want more beef with single coils. The diodes are of course a great place to change how it reacts to dynamics.
With the transistors, you can substitute many different types, but the main thing is that they really need to be high gain to work right in the circuit. I'm thinking Hfe of around 500 should probably be the minimum. Having high gain transistors is what allowed me to tailor the overall gain of the circuit with feedback easily and achieve stable low-noise results that are easily repeated.
MPSA18 would probably be a great substitute!