Man, if I had a quarter for each time I've posted that same question, I'd probably have a buck by now. There isn't much for the sachs specifically, I think there is an italian company that sells a piston/head kit-it's in the mopedarmy's links section.
I guess all that you can do is go with the normal stuff- open up the exhaust, fiddle with the carb, change sprocket sizes and make sure that everything works allright. I have a 78 Sachs Westlake that's currently under the wrench, so to speak. I'd like to find something myself, before I get it back together, BUT nobody has come forward with anything.
I hope you'll have better luck.