I would not mess with the motor.... You cant change the motor without first an upgrade to your controller. There is not many motor options out there for a PDS cart.
I would start with a good set of 4 gauge cables on the batteries.....
Then I would get a 400 Amp Alltrax Controller.
That will give you lots more torque and top speed.... That will hit your 20mph mark...
If you still want more after the controller and cables......... then you can upgrade it to 48 volts. You add 2 more batteries in the bag well. You will need a 48 volt charger and a heavy duty solenoid. The controller that I mentioned above is good for 36 or 48 volts....
Here is the controller that you need...
http://www.cartszone.com/catalog/cart09/490.htm