I had both the genoa and ballooner sheaves replaced when I bought my SM this past July before I sailed it up to the Chesapeake.  If I were you, I would change them both while you are up there doing it.  The boat was in Le Marin Marina in Martinique when I bought her so I just had Amel Martinique order the sheaves for me.   I checked the copy of my invoice and unfortunately all it said was "réas de tete de mat" or "masthead sheaves" so I do not have the part number or size to give you.  I can tell you that they cost 29.10 Euro each.  I would contact Maud in La Rochelle or Jean Collin in Martinique.  I might still have the old ones on the boat.  I will have a look this weekend when I am there and will report back if I do.