nick, until you ask more specific questions, i can't really help you. but ... on a minarelli engine, here are the things you can do to make a moped go faster (i think most of this is universal):
first, simple tune-up. be sure the moped runs as it should. that means especially proper timing. also that the muffler is not carbonized (i.e. clean it)
modifications can also be made. they include minor ones, such as chaning jet screws or carburetors (go from a 14.9 to a 14.12 and notice the huge difference). you can also get a larger manifold.
other major (or more dramatic/costly) modifications include chaning to a bigger front sproket (slower acceleration, but much faster top speed). also, a performance muffler (e.g. biturbo, polini, leo vinci).
finally, you can mess w/ pistons and cylinders. this can be something simple like making sure you have a true 50cc piston and cylinder (my bianchi was NOT and had to be modified to hit 30mph). or you can get a 70cc speed kit.
i think that pretty much covers it.