It's not a drill bit you (or your mechanic) needs, it's a tap. But as others have stated, it's definitely NOT a job you do with the head still on the block. Neither is installing a Heli-coil or other thread repair insert, which is what needs to be done by the way you describe the loose plug. Simply retapping the hole will not repair it. What it almost certainly WILL do is introduce a bunch of metal chips and dirt from the existing threads to the inside of your engine, and that will shortly introduce you to a very large and ugly repair bill. I would be extremely leery of any mechanic that wanted to attempt a repair with the head still in place on the engine.
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