Not an expert, but this is how I did:
Ignition: With one piston in top position, put a light mark on the flywheel, measure 2 3/4″ backwords and put a mark there (gives an ignition at 35 degrees).
You can now adjust the ignition with a stropelight at 2-3000 rpm, or with a lightbulp at still, but then you have to wedge the autoadvance to full.
I use modern two-stroke oil, and as it is quite thin, setting the oilpump is a problem, it is set to next to nothing (I always put 1% in the petrol) start in the high end and go down, oil consumption should be 1000-1200 mpg.
Lack of power could be the result of a leaking crankcase.
Someone once mentioned that riding with thin (modern) oil didn’t “seal” the glands (crankcase) and he lost power, returning to thick (50) oil, restored the power, but first after 50 or some miles.
The Birmingham engine is not much different to other Scott engines.
Have you had a look at: