The Spitfire's static thrust is approx. 3,000lbs but as airspeed increases that would fall away. The Merlin engine can produce 1,175 hp.
what do you mean the thrust would fall away? if it's a case of the planes airspeed affecting the engines air intake detrimentally then ok, otherwise do you mean it's acceleration reduces until it can no longer accelerate?
If it's the latter then the thrust remains the same but the air resistance increases exponentially with airspeed to the point where an engine producing 3000lbs thrust will not accelerate a plane that is experiencing 3000lbs of resistive force due to the air.
Like I said with the car, the thrust could be found if you can find what sum of the air and rolling resistance are at it's top speed.
The rolling resistance may be negligable since it increases linearly with speed (or maybe not due to increase in downforce as speed increases but probably still not massively) whereas the air resistance is exponential and would be the main factor in the resistance at high speeds, there's some ways of working it out by using the area of the sillouette of the front view of the car but I don't know how accurate they are.
EDIT: Sorry, on the thrust falling away, if it's a propeller plane and the drive to the propeller isn't geared then the thrust would vary with airspeed with respect to the power distribution curve of the engine.
To use my phone in the car I deleted all my German contacts, it's now Hans free.