There is no restrictor in or on the crank?has someone told you that?,the bike is limited to a certain rpm by the cdi coil and the transmission has a spacer to stop it gearing up and the exhaust has a heavily baffled silencer also the carb has a small venturi. best buy a big bore kit a carb kit and a secondary gear up kit to go with your expansion chamber pipe.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix motorcycles.