Higher voltage or more amps?
First calculate the torque you require - say, to get you, bike and battery up a 10% grade at 20km/h. Then plug in the torque constant for that motor to get the current you need. (If you don't have the torque constant you can calculate it from the speed constant Kv). Then compare that current with the motor's ratings, and your battery system etc. And update the question with your findings.

Then, realise that motor speed doesn't have to be the same as wheel speed, and consider a gearing system such as toothed belt drive... This would allow higher motor speeds and lower torque, for more reasonable voltages and currents.