Check the headers fit the manifolds and check the silencer brackets line up with the brackets on the bikes frame.
The 550 F series was fitted with a 4into1 as standard.
The 550K3 was fitted with a different style of 4into4 to that fitted to the 500 and earlier non UK spec 550's. Also if memory serves the K3 was fitted with an accelerater pump to no.2 carb.
Once you know the exhaust fits check the main jets on the carbs and have the carbs ballanced.
