Clunky gearbox

Hi I own a Honda Dax replica from Zhenhua, the gearbox of the engine has always felt clunky and loud when shifting gear. It has under 1000 km on it, so I doubt it is wear/tear. Im thinking about swapping the engine out for a new one and considering a 50cc Lifan engine, since I don't have license for anything bigger. Are there any japanese 4 stroke 50cc engines out there? And what are peoples experience with the Lifan engines?