The 550F1 has 35mm forks,the same as the 550F2 and the K3.
The headlights/brackets mentioned are such that all of the electrics are inside the shell of the K3, as against the F1/F2 models had the electrics squeezed into a little box just below the left front petrol tank rubber. As such K3 electrics won't fit behind the F1/F2 headlight shell.The K3 also comes with a Stanley H4 headlight unit, which, although not as good a Cibie direct replacement unit, was far superior to the unit fitted to the F1/F2.