Here I am going to start a post just for Garelli performance. There seems to be a wealth of info on the forum but it is all spread out and hard to find.
Polini 70cc Kit
This is a new cylinder and piston kit that allows the displacement to be bumped up to 70cc's. This will make a considerable performance gain, probably the most you can get out of your moped.
Here are some links to available kits:
1.) 1977 Mopeds - 
2.) Moped Cafe - 
3.) Federico Berkelaar - email@example.com <firstname.lastname@example.org> He is out of Argentina and is very good guy to do business with. He is also very quick on shipping.
Intake 20mm Larger intakes are needed when upgrading things like the polini 70cc kit. Works best when coupled with that kit and the carb listed below. 1.) 1977 Mopeds - 
Carbs Larger carbs are needed to provide the air required by adding performance parts. Works great when coupled with the 20mm intake above. 1.) 1977 Mopeds - Dellorto PHBG available in 17mm - 21mm 
Exhaust If your sucking in more air than you need to be able to spit it out with less back pressure. The only kit i've found just for the garelli is the polini kit but I have also read it sucks. You can basically fit any exhaust you want if you can weld or know someone that can. Maybe everybody could give there opinion on the different exhausts they've used. Also when deciding on a kit that has directional features such as the Technigas kit use the one for a PUCH it is on the right side for a Garelli.
Here are the available exhausts you can use:
1.) Bi-turbo - 
3.) Techno Circuit ( looks like it will fit with a little modifying - 
4.) Techno estoril - 
Ok so you get the idea anyone can fit with mods and a PUCH is the right side.