Look at the piston's dome. it will give you a number, such as 38,8 or 3-,-. That is your bore in mm.
Sometimes they do not have numbers, in which case, the only way to know is to measure it.
Add a 15:15 or a 16:16, Mamaba (or Estoril, though I personally have not tried this pipe YET) with a 38,8mm bore and a 11 X 44 gearing, and you should be somewhere around 35ish. Adjust your gearing, and you can squeeze more out of it. Do some porting, and you could probably get 'er doing 40.
I find Minarelli's aren't the fastest stock bikes out there, but they have a lot of torque/low end. If you want fast with easy and cheap mods, get a Puch.
Oh yea, and pretty much what everyone else said. Minarelli's can be funny about their air ratios.