It baffles me too, it's essential for that to remain open. After reading several accounts in here where after a 20 year time period, people have gone back inside their pristinely overhauled tractors to find two inches of crud laying in the bottom of the crankcase AGAIN, I'm thinking we all need to open these these things up every ten years or so to clean it out again AND knock that hole open just to be sure the top end gets the oil it needs. They report that they are using high grade Diesel motor oil and changing filters often too, so the crud build up shouldn't be happening, maybe it's just the nature of the beast to do this? At any rate, that's my plan even though I remain baffled. I also went to the ACE hardware store and bought an extra long 1/16" drill bit and opened that hole up a notch, now I can use the drill to open it back up with too.
But mine gets it's rocker arm oil from another source, you might wind up with a bit of oil smoke if you go heavy with oil on the valve stems. Mine only oils the exhaust rockers and they splash enough onto the intakes that they work fine, but everyone that 'improves' that oiling system to put oil on the intakes too then complains about the constant blue smoke they get for doing that and all to often revert back to get rid of it. Just a heads up if you wanted to 'improve' your oil system while you are in there.