I once had the same problem with a Cat 3406B engine in a Freightliner truck. The hole in that case was about 1" diameter and was for an alternate dipstick location, with a rubber plug that apparently had fallen inside the base. I bought a new plug from Cat, but it had to be installed from the inside out (had a ridge on the inner end). I really didn't want to drop the base pan to do that and I guessed the old plug couldn't do any damage in there. So I bought a generic adjustable compression plug that tightens up with a screw (something like an old time Thermos stopper), installed it from the outside and never had a problem with it.
