max carb you can fit without being a wizard is a dellorto 13.13...vespas are a different animal..
you are in a pickle here man. even if you got all the right go fast parts, tuning is gonna be a process....i wouldnt advise modding you only means of transportation...
for a first step you should look for a good expansion chamber exhaust. your carb should be a dellorto Sha. (hopefully im not an idiot and they still used them in 87)...buy a few jets in larger sizes you have and be golden...should get you a few mph and better acceleration. shouldnt take more than a couple hours to get it right...
somone may be willing to make a deal and ship you a buncha thingys in one package,,,