If the hole is deep enough the new oversized drill bit will follow the hole, same for the tap. You could first use a drill bit that fits into the existing hole then change up to the size you want. If for example your enlarging the port hole size then it really wont make much differance as the wall is pretty thin. If your using just a drill press then you can use "V" blocks, if you have a close fitting drill bit you can align it that way. Gage pins are another way to center an existing hole.