We should make a rule around here, that people should read their speed on GPS, because that isnt possible.. When the speedometer is at 50 mph, it will show between 10 and 25% wrongly, and theres nothing much to do about it..
About getting more speed: it wont be possible.. Around or a little lower than 50 mph is the maximum for 9.000 RPM with the standart gearing, and some (me included) have achieved this with the 49.5 ccm standart cylinder.
When your CDI is cut, it will max out at 9k rpm's, and then only ignite every second time (which feels like someone is rapidly turning your key to the off- and back to on-position), until the rpm's are lower than 9k again.
A bigger engine wont give you higher rpm's and therefore not higher speed, if you are already maxed. On the other hand, a nice quality 80 ccm DOES almost double your horsepowers, and makes it able for you to run with a higher gearing. I, myself has got the standart gearing, so i dont know how much a naraku secondary gearing will give you, but it might give some..
Also if you can find an unrestricted CDI with advanced igniting, you can gain some if it lets you pass into higher RPM's, but be aware, that higher RPM's will put even more stress on the engine parts, more heat, and at some point the valves wont have time to close before they will be opened again. Driving with high rpm's is both damaging for the engine and stupid, as the max horsepower occures at 7.5 RPM's, so it is limited how much you will gain..
Keep in mind, that cars cylinders is 4x300 ccm, and even though their engines are much more effective than ours, they arent able to acheive more than 180 km/h with an engine 24 times as big as a standart agility.
So the conclution will be: your speedometer isnt accurate, so please read your speed elsewere before posting, and if you want more speed, get a 125 ccm engine with an even higher gearing. Or be satisfied with what you've got now