This figure shows a curve for a typical DC motor. Not all motors follow this curve.
http://lancet.mit.edu/motors/colorTS2.jpg
Notice the maximum power. It is a product of the torque and speed of the motor, and that is what the motor can output and no more.
At stall torque, where the motor is not rotating, there would be a maximum current. You would not want to "run" your motor there for too long as it would overheat quite quickly.
The only way to increase the power of the motor, would be to increase the voltage to the windings and rotor. In which case your power supply might have to be upgraded ( unless you have a variable power supply that can handle the extra voltage and current ), which would cost probably the same if not more than a more powerful motor.
You could take your motor apart and do some re-winding of the stator and rotor, but unless you know what you are doing that is not recommended for continious safe service -experimentation possibly if that is your goal, but limit youself to small hobby motors of 6v or less.
As for an AC motor running off the mains, let's say the N.American 120v ac 60 Hz , you are stuck with 120 v ac 60 Hz. And don't try plugging a 120v motor into the 220v split because you will not have a motor for too long.