https://www.deraceheldenvanweleer.nl/index.php?topic=1659 Check out this amazing two-cylinder 50cc race engine built by some Dutch fellow named Nico Classen (Classen Racing Motors or CRM). You have to register for the "Heroes of Yesterday" forum in order to see all the pictures and use Google Translate to understand what they're saying, but I promise you it is worth it. Two 25cc cylinders for 50cc total displacement, 20,000rpm limit, rotary disc valve induction, 8 speed transmission.