There are some cars that look OK resto-modified. Like Mike says, air cooled Porsche 911's are prime candidates. Those cars actually look pretty good when modified correctly. And, some companies, like Singer, charge hundreds of thousands of dollars for their modified 964's.
Mercedes SL's, on the other hand, are so beautiful that external modifications often "cheapen" the look of the car. I don't even like those chrome eyebrows on the wheel well lips. Wire wheels, added chrome, etc. is not my thing. These cars look great the way they left the factory.
My old acquaintance from MA, Bob Hatch, tried selling SL's with V8's in them. I believe he even tried to electrify a 280SL. Externally, those cars looked bone stock. Performance was greatly enhanced. That being said, I like original.