If you're going to drill, use the smallest bit you have. Then work up. I believe that's a blind hole, so don't drill all the way through or you may have a real leak. I hear what's being said about welding onto the stub, but is it safe to do this on an aluminum casting? If the cover is damaged in the process, you're done.

The timing chain cover is machined as a unit with the block. If you end up needing a new cover, it's the end of the road. You're not going to pull the motor to deck the timing cover.

If form a gasket doesn't work, at least you're no worse off.