I am assuming that your rudder shaft is still attached to your rudder correct? That's expensive shipping
It does appear to be crevice corrosion. Also assuming that your rudder shaft is hollow. If so the repair
would be to cut the shaft in the center of the corrosion, grind back any punky material, fit a machined spline of at least the same thickness as the rudder shaft, and weld everything back together keeping the length of the shaft the same as the original. Can be done locally by a welder equipped to weld SS. Do not assume the shaft is 316 it could be 304.
It would be best practice to passivate the shaft after all welding, grinding and machining.
Who mic'd the shaft? It may be 60mm but it could also be 2" pipe (hopefully schedule 80). 60mm = 2.362", the OD of 2" pipe is 2.375".
If it's solid, same method but instead of using a spline the shaft will have to be bored on both sides, pinned and welded. You will probably need a short length of 60mm rod to replace the punky material that has corroded.