get a gas powered one
Unlock the gas on the handlebars
Go downhill
Yes it will make the scooter go faster, but you could run the chance of blowing parts in the moter if you do so
Either increase the size of the front sprocket or decrease the size of the rear sprocket. This will change the gear ratio and make the scooter faster. Although you need to kick harder to start the scooter. Also, I do not know of any sprocket mods available online so you would probably have to get it custom made which may be pricey.
but a engine in it
It depends what kind. With the ones with the raised back you could put a chainsaw motor on it. 45+
the brake
An improved flow of air and fuel mix will make the scooter faster. You can do this with an aftermarket exhaust.
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Loosen the headset a bit.
The average electric scooter will not go at speeds that are greater then thirty of forty miles per hour. Make sure that you wear protective gear to make sure that you are extra safe.
Here's a basic wiring diagram for an electric scooter, bike, or go-kart that shows how to connect all of the parts. The batteries that will power it will be seen in the following photographs. The wiring is illogical. Because this is a 48v 1600watt scooter, I need to locate the wiring. Many separate cells are joined together to make electric scooter battery packs.