I'm not able to do custom work anymore, but it shouldn't be very difficult for custom builders who know their stuff.
[I haven't played through one of these or even heard one, just looking at the info available online. ]
There's nothing unobtainable inside there. I doubt the transistor is finicky. The usual NPN suspects should do. The only tricky thing, parts-wise, is that this kind of circuit requires a specific type of opamp to do its thing. The common modern opamps will not work, but fortunately the ones needed are still plentiful.
Don't know if you've tried it already, but I believe the EHX Doctor Q might be just about the same circuit. And it's already in a nice small box with LED and true bypass.
