All I can think of without stroking it is adding pod filters, jet kits, and exhaust mods. You could always lose some weight on the bike in places I guess.
Side from from these I wouldn't get to deep into the engine trying to get more out of it. From my experience the more "tuned" an engine is the less reliable it becomes. I had an old Vespa that I put a 210cc conversion in (198cc stock), 8% larger main gear, Bored out the air/gas ducts, installed a 30mm side mount carb and an expansion exhaust. This thing was fast. Scary fast in the power ban. But I had to rebuild it almost every other year. The last time it broke I blew up the clutch basket. THEN I sold it.
The first Vespa I owned I did nothing to, not even change the oil, and it lasted years while commuting 50miles a day on it.
