Basically nothing Jerry said is of relevanance to your situation because you have a two-stroke scooter, based on the minarelli engine(i think). The metropolitan is a four stroke.
You can get a Technigas next R here break>
You're going to want to get a high-flow air filter and rejet the carburetor when you install the pipe. You're on your own with the jetting, since i don't know which carb you have, try any decent scooter shop for info and jets.
There ought to be a washer between the two cheeks of the variator, but maybe they didn't need that restriction in your area. A good variator kit will increase your low end and midrange, and take real advantage of that exhaust upgrade. check out this doppler system break>
I think you're looking at about 200 pounds for all those upgrades, which is not too shabby at all.