To keep it simple and stick to the basic question, let's assume that you actually got rated power in ISA conditions when the engine was new.
The short answer is that no you will not get the same power output if the compression has reduced with age.
Please note that I said "compression" rather than "compression ratio" because "compression ratio" is not a ratio of pressures, but is a ratio of volumes (Total volume / Clearance volume).
As engines get older their valves and piston rings become leakier, so the pressure at the end of the compression stroke decreases. This in turn reduces the pressure during the power stroke, so the power output decreases as engines get older.
We could say that the power output is equal to the power generated at the faces of the pistons minus the power that is lost transmitting this power to the end of the output shaft.
Power generated at the faces of the pistons is called Indicated Horsepower (IHP).
IHP = (Mean cylinder Pressure x Piston area x Stroke length x Number of pistons) 33000
Looking at the equation we can see that if cylinder pressure decreases then so does IHP.
It is a reasonable assumption (but by no means certain) that all of the other factors in the equation will remain constant as the engine gets older.