Hi,
You should be able to use the existing settings to do this. The joystick map converts the raw input to the engine proportion. If you use the settings editor, you should be able to flip the signs on the joystick_map(*,2) entries. So for example, you would change the from the default:
joystick_map(1,1)=-1
joystick_map(1,2)=-1
joystick_map(2,1)=-0.5
joystick_map(2,2)=-0.5
to:
joystick_map(1,1)=-1
joystick_map(1,2)=1
joystick_map(2,1)=-0.5
joystick_map(2,2)=0.5
Separately, I have added a feature to swap the rudder direction if needed, which will be in the next release (or is in the code/binaries on github), but I don't think you need this.