Have a look at this post: https://www.scootershack.co.uk/threads/derestriction-of-honda-vision-nsc-50-4-stroke.29395/
first thing to check is the carburetor, it might have a plate in the mouth of the carb that restricts airflow, when you remove the plate, you need to upjet, like 10-15% because the engine can flow more air and fuel through it. you could also check the exhaust, it might have a restriction plate or a knob welded to the header. you would need a dremel tool and maybe some files to remove that. the cdi might be restricted too, the cdi is like a small computer that controls your ignition, it might be restricted to a specific rpm, you cant derestrict that but you could buy a new cdi, the cheap chinese "new racing" cdis work quite well.
