Started the motor for the first time today. It roared into life with relative ease. I soon realised it was starving for air, so tore it down again looking for reasons why. Didn't take long to see why, the velocity stack had an internal diameter of 15.4mm, but the carb internal diameter is 19mm!
I bored out the inside, then while it was dialed true in the lathe, thought I'd contour the shape to aid flow. Pretty happy with how it turned out.