Thanks all, I'm tempted to try hammering out the sealing edge with some suitable tool that won't deform the cup and see if that produces any results.
The plug is just a seal according to the parts list.
It appears there is a slight bend upward on the left side of the rear drawbar slide. When I got the tractor, the drawbar was not installed. It does not look like it has ever been used. However, one of the four bolts attaching the drawbar carrier to the bottom of the trans. housing was partially out of the hole and the threads boogered. Dealer didn't have the drawbar pin; It was a king's ransome anyway. So I'm making one of those. Since I don't see any obvious damage to the Drawbar carrier, I'm having a hard time imagining a blow hard enough to deform the case and make the plug leak. Besides, that is supposed to be the toughest part of the tractor, right?