you can get an expandable rubber plug at Home Depot or at the Auto Parts store. Size is just over 1" if I remember correctly. You want the rubber plug that has the bolt thru the middle. Clean the oil out of the hole with brake claner and then put some 1104 ThreeBond on the plug. Resist the temptation to over tighten the plug. Just get it a titch more than snug. The tighter you make it the more likely it is to want to pop back out since the rubber area isn't exactly flat along the length of the plug when it is expanded. If you don't have 1104, then any good gasket sealer will work fine.
Tracy