I've got a 1.8 Pinto which is in good condition. Is there really a huge difference between 1.8 and 2.0 (apart from the obvious 200cc). I've got the head off and the valve sizes and ports look the same to me as the 2.0. If I put a mild cam, better carb and 4:1 exhaust in both would there be a big difference in bhp/torque?

200cc is big enough to make a difference
As the engine sizes are the same replacing the 1.8 with a 2.0 is a very easy and cheap 17bhp.
Adding a mild can will give you mild increase in bhp & torque but approximately 10% less than if you put the same cam in a 2.0
One of the biggest advantages of a 2.0 over a 1.8 is the ability to put the 2.0 injection head on it which has better flowing inlet ports and the injection rods which are stronger.