Thank you for providing so much information that makes my job easy here. For a 20 MPH top speed with the ability to climb 11% grades I recommend using a 1000 Watt motor instead of 700 Watts.
Here is a gear ratio calculation using one of our 1000 Watt 3000 RPM motors along with a 44 tooth axle/wheel sprocket and a 9 inch wheel.
As you can see the top speed is 20 MPH with this setup.
We can make parts recommendations for this modification or put together a parts kit for it with matching and labeled connectors.
Please let us know if you have any questions.