The Honda rebuilt kit should still available, part #45530-377-305 - Master Cylinder Set. I don't know why it doesn't show up on the parts fiche for the 550K. The master cylinder, part #45500-341-771, cross references to a bunch of the 750's (among other things) and the rebuild kit shows up on their fiche.
Has anybody ordered one of those for their 550K? If so, any issues?
~john