Short answer: When you increase cc, you have to upjet. Change to a freer flowing pipe, upjet again. As been already mentioned, a bigger bore will not increase your top speed, it will just get you there faster. If you want higher top speed, then you have to change the gearing. You can keep your stock carb during all this. With a bigger bore, you should have the power to handle the gear changes. It's fun to mess around and mod the 50cc scoots - in Hawaii they get the older Honda dio engines up to 70mph. But eventually you realize if you want to go faster, it works out better just to get a bigger scoot.