I am not sure how much material "metal" you have all the way around the existing hole so any advice is conjecture at best. My best guess is that it is time for you to find the smallest possible bolt that is still larger than the previous one and tap that hole out to match it. I am fairly certain that you will leak substantial amounts of oil from the cover with only two bolts though. Very sorry that this has happened to you. Are you positive the case threads are fouled? On the TW the bolt threads typically give way before the case threads.