check the resources section and look for "how to fix your moped" in the articles. it's also known as "fred's guide". it's great.
your spark plug is fouling because of too much oil. the proper fuel/oil mixture for a minarelli engine (what the cimatti uses) is about 2.8oz to the gallon. too rich will foul the spark plug. also, i recommend bosch platinum 4214. it's the best.
how fast does your city bike go? 25mph is a very good speed. so don't be disappointed if it does 20-25mph. it can do more, of course, but at a huge cost in performance upgrades and such.
cleaning the carb is easy, just basically take it apart carefully and use carb cleaner on all the internal parts. the whole carb only has four screws (two top, two bottom) and then a screw to hold it onto the manifold and another to hold the airbox onto it.
an inline fuel filter goes a long long way to helping you out. you can get one at pretty much any hardware store. you can also get filter on your petcock (fuel valve) and there is one in the carb.