Yeah, the 650 made 73 HP @ the CRANK. The 550 had 49 HP at the crank. (subtract about 20% for rear wheel HP) You are not going to get that much, just by swapping heads. There's a matter of an 18% increase in displacement. I am not sure what carbs would go with that kind of change, or if the 650 head would even fit. I do know that heads from a post 1980 550 won't work, because the carb spacing changed when they went to CV carbs...(which BTW are extremely difficult to tune without an air box). And I would assume you would need to modify or eliminate the air box with a carb change. And you would, at least need a rack to match the 650 head that you could install what ever carbs you went with.