https://www.amazon.com/Electric-Removable-Lithium-Ion-Adjustable-Handlebars/dp/B01KVAIXY8
You have limited options due to the following reasons:
1. The motor is rated at 250 watts
2. The gearing of the motor may prevent the motor from spinning faster than the factory spec.
However, there is one option that is very easy to do, but it will cost you money ($399.95 to be specific) and you run the risk of burning your controller. And that is by over volting it (the hot rodding solution in ebikes). Increased voltage enables the electric motor to turn faster.
https://lunacycle.com/48v-panasonic-11-5ah-or-13-5ah-bottle-battery/
If you add up the total cost plus the risk of destroying your ebike, then tthe more decent $1200-1500 ebikes make more sense.