Can AC fan or motor be upgraded?
It's possible to change the motor, but doing so may not change anything. In fact, blindly changing the motor could actually cause the system to perform worse. 

Without knowing a ton more about the system, it's impossible to diagnose the actual problem.  It could very well be poor duct design, an undersized system, or a whole host of other problems.  The best bet is to get the system assessed by a local reputable HVAC company.