Wingspan, Absolutely beautiful project! One of my all time favorite scooters, I just bought one yesterday (AT LAST!!), which caused me to find your project!
Before you condemn the needle/seat in your leaky carb, examine very carefully the actuating arm where the float(s) contacts it. It must be PERFECTLY smooth and flat. The floats impart only a very tiny force on the arm, and it doesn't take much to impede the force needed to completely close the needle/seat. The floats move in a straight line on their guide pins, and the arms swing in an arc. Those little steel pins on the floats sit at nearly the exact same spot all the time on the brass arms while they let fuel dribble in to the bowl, and the constant vibration wears a tiny notch in the soft brass arm just enough to eventually make the needle not quite seal, or at least become maddeningly unreliable! Some arms can be inverted, but if these can't, sand or file the area flat, polish smooth and reset the float level. I've replaced a lot of needle/seat assemblies, and that actually fixed the problem a lot of times, but I realize now that most of the time it was probably just disassembly and reassembly with new parts that landed the float/notch alignment a hair off and the seal worked, lending credibility to the belief that the needle/seat was bad, when actually it was that arm. I don't think I have needed a single needle/seat assembly since I figured this out. Hope this helps and gets you running for the weekend!
again, great work!
John