Altering port timing effectively increases torque at higher or lower RPM.  Thus making it a higher or lower RPM engine.  So you are not likely to make an engine stronger at a specific RPM.  To do this you would need to decrease friction, increase breathing at the specific RPM, increase the effective compression, or increase the potency of the fuel.  You are not going to make significant improvements in decreasing friction in a ball bearing engine.  You can improve breathing by increasing the carb size, improve flow to the ports, ie larger bore hole in the crank, or smoother flow from carb to port, larger ports in the case, or wider ports in the sleeve.  But these will be minor in most engines.  You can increase both the breathing and the effective compression by installing a tuned pipe.  And the best way to increase potency of the fuel is to increase the nitro percentage.