According to our Motor Power Calculator, the fastest a 650 Watt brushed motor could propel a scooter or bike on flat ground is around 22 MPH.
This is mostly due to the amount of air resistance (drag) that the bike will encounter at that speed.
Due to this calculation we do not recommend gearing the bike over 22 MPH otherwise the motor will lug and produce less horsepower than it is capable of which will reduce the top speed even more.
To obtain a top speed of around 26 MPH a 1000 Watt or higher motor would be needed.
We have many modification kits available for the Razor MX650 with 1800 Watt motors that are capable of speeds over 30 miles per hour. They can be found on this page: https://cart.electricscooterparts.com/store/Search.aspx?SearchTerms=razor+dirt+bike+modification+kit