I think the answer to that depends on how good of shape your transom is in. I'm assuming that your transom core is wood, correct? If so, you need to first determine if there is any rot around the drain where the leak occurs. If so, the rotted or compromised wood will need to be removed and the whole area patched, which will then allow you to re-drill and re-sleeve the drain. I've not ever done this myself, but have seen others do it. You need to pull the drain sleeve out and inspect the transome material around the leak area. If all is well, or there is minimum damage to the transom, use some "Get Rot" to strengthen the wood if necessary, then fill with a good two-part marine epoxy. Re-drill and re-sleeve and seal the sleeve with marine epoxy or 3M 5200.