Kinetic magnums are the best stock vespa
Keep the stock pipe, it's actually great. So is the stock top end. The carb should be a 12:12 which is as good as it gets without splitting your cases to port to allow flow for a larger carb.
What you want is a variator. On a stock kinetic magnum, with nothing but an upjet and a variator, I hit 47. Best stock bike ever