I think if you cut the header at the base where it meets the exhaust flange you can rotate it and re-weld it back together. after you do that, you can weld up any Vespa pipe you want. I'm going to adapt my estoril that I already have on there.
I thought about the polini reed block too. Unfortunately, the bolt pattern forces it to face forward into the tank on a ciao. I think you could point the inlet straight up and then use some bendy hose and clamps to get a carb on there. OOOOOOR, your could fill in the reed block holes with JB weld and then redrill/tap a bolt pattern the lets you turn the reed block around. This is all theory, I'll let y'all know when I have proof.