WOW, you really are missing a lot of stuff. Unfortunately, you will probably make ebay rich, buying everything you will need. There are better mopeds around. Try to find a Puch or Motobecane that is mostly complete. Fixup cost will be much lower. For what you will spend to get that Garelli running, you could easily get a recent Kinetic or an almost new Tomos.
If you must try to repair it, take lots of pictures during and before disassembling anything. Best of Luck.