You will have to change the aircraft cfg file for one and you can also edit the .AIR file as well there is a section in the aircraft cfg file that controls the power that a engine will put out wheather it is a jet or a prop type engine. As to how much you can effect the actual power of the engine I am not sure I have never played around with the settings but I am sure someone else can give you ideas as to what the parameters should be. 8)
The aircraft cfg file controls most of how a plane handles but the .Air file also plays a important role AIRED is a program that allows you edit the air file and the aircraft cfg file can be edited using notepad or wordpad as it is just a .txt file.
most designers put in good comments in the aircraft cfg file and you will find several sections in it that controls specific functions in the aircraft . You just need to find the section that you want to edit make the changes and save the file.  OH BY THE WAY MAKE SURE YOU BACK UP ANY FILE FIRST  before you make any changes so you have something to go back to if it blows up in your face so to speak. 8)

I actually just finished doing something like that. Check out my Cessna Sport. The roll is controlled by aileron up limit and aileron down limit in the aircraft file power would be under the  engine specs or engine performance and I'm not sure about the overspeed thing