from what I understand the main difference is in the reeds and carb, the 6hp has 14mm carb and 2 pedal reed, the 9hp has 19mm carb and 4 pedal reed, I think they use the same head and cylinder. the difference between the lower 3hp cyclinder and 6hp cyclinder is the amount and size of transfer ports. correct me if I'm wrong.
I know I didn't mention the CDI and magneto
I don't know of any aftermarket kits and they're probably from China. I would stay with Morini, Made in Italy.
Did you cantact Herdan?
personally I haven't ported a Morini yet......