I think Cees is right. Some people may not be interested at all in an SL that has a 980 engine. That would, of course drive the price down, without really being able to put a number on it. The same thing applies to a car that has been painted a different color than it was, one that had an aftermarket AC installed, rust repair work, etc. I would say that a person that values a very correct 280SL the non-correct engine (or paint job) might still consider the car with a 10% to 15% discount, but it is a very subjective decision.
Best, Tom