spring tuning is done after the shoes are all lightened. you basically mark each clutch shoe at the same place and use a fishing scale to pull each arm out to the mark, making sure it takes the same amount of force to pull the arms. this allows for the clutch to engage equally and smoothly.
a 21mm dellorto will give you less restriction on your top end, but a parlini and techno circuit shouldn't really rev past 9k or so, so its not like a smaller carb couldn't do the job. when you start getting into crazy shit, like MLM/MMM/Simonini/Weak-ends pipes and Polini/Athena/Gila builds, the 21mm carb will do justice. but till then you really don't need anything crazy.
you will want a clutch brace though. http://www.1977mopeds.com/product/50116/Mike-Thomas-Puch-3-Shoe-Clutch-Brace/ or http://www.1977mopeds.com/product/1048/MLM-Puch-E50-Moped-3-Shoe-Clutch-Brace/ both work.
there is alot to learn to building an actual blaster of a ped that is fully tuned. it is the best though.