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Going back to modding your 50cc engine, if you put on a 50mm cylinder kit and a stroker crank, you have a 125cc engine with 50cc engine number, vin and any other reference to it will all show 50cc. Put in a 125 engine and the engine number will be a 125 engine number. If anyone wants to check, you will be gone for licencing and registration. Though in Australia, they must be registered and you must have a licence, a car licence will cover it.