Early 550ks (K0-K2 sold in the US) are like the 500s with the smaller 500 tank. The K3 and 1978K are like the F at a glance but the footrests and rear brake lever and positions are different. Think 550F and 550K3, 1978K side panel mounts are the same, but different to K0-K2. As said as long as you replace tank and seat as a set you are OK. The swept 550F pipes (if you can find any) would probably fit K3 and 1978K, but may need a special bracket for any silencer.