I went thru this same ordeal. The solution is get cylinder kit. The lower end of the piston travel (bottom end) wears down faster than the compression end. NO suction of fuel/air mix, when you put a load on the engine. I had advice from some of the best Vespa mechanics. I bought for treats the 43mm oylimpia kit with head. Note piston ARROW goes to the intake side. Torque to specs and lap the decompression valve.
Plug ngk b6hs .020 gap, points a tight ,016 gap, time at 19 degrees, new carb shim a 66 main jet on a 13/13 Dellorto Sha and a promo circuit pipe. Andy and the Zeros helped with advice, pat splat and monte. Dan double 1 was spot on about low suction. I checked ring gap, it was out of spec. use your std crank. 38 to 40mph. I used a hi flow air filter and a clutch pad upgrade on a single speed.