First a disclaimer: I don't own this machine and have never seen what you're describing.
But from your description, it sounds like a welch plug (a.k.a. freeze plug) sealing a hole bored to house or allow installation of a bearing. For other readers, a welch plug looks like a smaller version of a jar lid, but made of heavier metal and slightly oversized for the hole. It is driven into the hole with the cavity side out, and usually some sealant around the perimeter.
If your plug is a welch plug you should be able to remove and replace it from the outside. Try drilling a hole in it, doing your best to keep the "shavings" out of the hole. Be sure to stop drilling as soon as you're through the plug. Drive a sheet metal screw into the hole and use that to try to pry it out. Or try driving a punch close to one side to make the plug pivot in the hole.
If you have the means to accurately measure the inside diameter of the hole (ideally with calipers, but a good ruler might work), you should be able to buy a replacement plug from NAPA or other car parts place. Put some sealant on the perimeter of the plug and, using a socket from your ratchet/socket set which is close to the size of the plug, gently drive it into place with a hammer.