The most accurate way is to take precise measurements of the old sheaves before removing them from the mast. This includes the sheave diameter, groove diameter (if applicable for wire rope), sheave width, and pin diameter. If you know the manufacturer of your boat, reach out to their customer service department. They might have spare parts catalogs or technical drawings that include the halyard sheave specifications. Stainless steel with ball bearings sounds like a great choice for durability and low friction. Make sure the chosen stainless steel grade is appropriate for marine use to withstand corrosion.