A 500 Watt motor is good for a 20 MPH top speed. A 750 Watt motor would be better for a 20 MPH top speed if the bike needs to climb hills.
Along with installing a more powerful motor the bike's original 80 tooth sprocket could be replaced with a 55 tooth sprocket to increase the top speed to 20 MPH.
The 500 and 750 Watt motors are not a 100% direct fit for the MX350 and the frame's motor mounting plate may need new holes drilled or other minor modification to install the new motor.
We can make a parts list or a kit with matching and labeled connectors for the modification. Before we do this though could you please let us know if you prefer the 500 or 750 Watt motor?
Please let us know if you have any questions.