Buy New Ones
I would buy new ones. I've tried drilling and putting new sheaves in and it just doesn't seem to work quite right. I bought all new Garhauer blocks for my boat and they don't cost a whole lot more than the sheaves would. I would keep the old ones and refurbish them for a back up block if they are ever needed, but the smart way to go would be to replace them with new. If you do decide to buy Garhauer, Plan on plenty of time to get the parts, they are really busy this time of year.