Yamaha Motor Scooters Japan

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I bought a mint yamaha 50cc scooter its a surplus of Japan.My question is where can I find the engine number This thing.Ive searched everywhere, but I never really found it.Can someone help me? Please explain in detail coz I am not a technical man on motorcycles. This is a 2-stroke engine.
Always give year make, engine size and model # when asking questions. We have all this information to make an informed response. Different models will be the engine number on different locations. Why do you want the engine number? The bike is identified by the frame number. Look at the top of the crankcase behind the cylinder or on the side of the crankcase. Or contact on Yamaha and ask them - http://www.yamaha-motor.com/sport/contactus/home.aspx