if you have the 1/c engine and you're only going 20 mph you need to remove some restrictions. there is some good info here on changing carbs break>
check the exhaust header, there are 2 sizes that they came with; a small restrictive one and a larger less restrictive one. if you have the smaller one, order the larger one from handybikes for.... i think 15 plus shipping.
take the exhaust off and try and clean it out. take it off, take it apart, get some carb cleaner goin, maybe a wire brush if you have one, just whatever it takes to get that carbon build up out.
if your carb has a wooden float, that's hilarious.
if you're a little creative you can throw a delorto on that bitch.
do that, plus an expansion chamber, you should be goin 30-35.
whatever you decide to do, will require patience, creativity, patience, getting dirty, patience, etc etc etc. unlike a puch or a tomos, there really isn't a lot of bolt on stuff for the sachs. you've got to fabricate stuff yourself from other existing parts and materials. but it is so worth it. everyone sleeps on sachs mopeds b/c they assume the clutch is janky and there is a real small performance parts market for sachs. but my STOCK mopet does 27-32 flat, and the only mod i've done is to put the bigger exhaust header on it.