Zündapp KS 80 to 125cc???

Hi, have a KS80 from Zündapp, but the taxes and insurance contributions are quite happy. I heard that you can open the KS 80 to 125ccm, and then register it at 125ccm??? What do you think, how is it worth it, or too expensive??? so please help… many thanks for your trouble… Mfg Dax