Moped Army

I want to work on my Vespa Grande project this winter and I was wondering what the best pipe would be. I'm pretty sure I want a top end pipe since the variator will give plenty of torque. It's going to be moderately modified with:

64cc kit
malossi variator
performance crank
13.13 dellorto
high flow filter
maybe even the rear gears for the gear box.

I already have all of those parts ecept the gears and I need to get a pipe but I don't want to spend the dough on a mediocre pipe.