hmmm. i don't think the serious modifications to the engine, moped frame and your wallet would be worth the increase in performance. you will also have to beef up the cady's clutch and change the gear ratio to make it go faster. i have modded my Cady by putting a pipe on it and removing the airbox. and it goes near 30mph. see attached image to see the modifications.
also the Cady was never meant to go fast. its simple designed frame might not take the added speed. also the brakes aren't very good on the Cady.