I’m going to fix it but Still want to buy the back up plugs just to have on the boatIf that sea cock splits, you won't be able to plug it from the inside. In addition, the hull fitting most likely is a bronze strainer type of fitting which also can't be plugged easily.
In general, you can get an assortment of wooden plugs at a marine store. There are plastic typed that are usable for a range of hole openings.