Before you put the front cover on the engine, check to see if the oil passage plug is there, it is behind where the camshaft gear would be.
Below is a pic from TM Tractor showing it, it is to the upper left of the camshaft hole, left in the picture.
There are other plugs on the engine, one is in the area with the valve springs.
The plug Glen mentioned above is the only 1/4" NPT oil passage plug in the engine. All the rest are 1/8" NPT.