My 2001 MKll also has limber holes. When I first got Shamrock many of the limber holes were pluged with wood chips, small bitts of fiberglass, ect. Took a couple of cleanings for them to stay clear. Don't know the O-ring size. If it wasn't lost, there should have been a plug for the knot meter. It will have an O-ring and and the threaded locking ring. Plan #2 (if you have or can borrow a set of calipers) measure the hole inside diameter and the outside diameter of the O-ring groove. A good hardware store, chandlery, or industrial supply will be able to size it for you. I'd be surprised if it was more than 1$.
Hope this helps,
Jim