The boost circuit is very simple. It seems very peculiar that you can pick a signal up at the collector of that transistor but you cannot follow it through C15 to the output pot and out of the pedal. Sounds like something is wrong there - maybe you have a short somewhere.
The reason I flagged this up is when I trace the signal through a transistor on the OD side it has negligible effect on the sound however I described the complete signal degradation between the poles of Q3. I'll work out something. Quite tempted to swap out the silicon based boost for MOSFET anyway so maybe in that upgrade I'll fix the issue.
Any general purpose NPN silicon transistor in that position will give you the same results (although you will need one with the same connections to just drop straight in). The MPSA18 is a more recent design with a high Hfe but the gain of the stage is determined by the ratio of the emitter to collector resistors. If you order the same transistor you know it will drop right in.
Yeah, I've ordered some straight swaps and also the parts for the MOSFET upgrade so I can see how it goes. In reality I'd like as cleaner boost as possible. I tend to use a strat with single-coils which I believe are rather on the low-output side.
12k or maybe even greater will do fine with the high brightness LEDs. I can't say what brightness you would prefer.
I've ordered a few of each so I guess it's just plug and test.