Replace the stock air filter with an aftermarket filter. This is one of the best upgrades you can do, as it is cheap, easy and effective. With a $5 air filter upgrade, you can expect to gain a bit on your top speed and overall better performance from your moped on hills and tough terrain.
Fill your tank with premium fuel instead of regular gas. The high-octane mixture can increase your Ruckus's performance, and it might increase your top speed.
Replace your Honda Ruckus's stock exhaust with an aftermarket one. This upgrade can increase its top speed by at least 3 mph.
Purchase a big carburetor kit, making sure to get the one that is specific to your particular Ruckus model. The "big carb" kit will include a larger piston head, which can increase speed by 4 mph or more. Although these usually cost a couple of hundred dollars, the money is well spent.