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As much as it hurts to hear, you really need to split the case if you're going to drill and tap. Engines aren't designed to have metal free-floating in them, and that's exactly what you're going to have if you drill and tap with case assembled.
To seal it, the only thing I can think of that would last would be some JB Weld.
Clean, sand, apply, and pray to the JB gods that it holds till you're in the mood for a teardown.
Good luck, let us know what you do.
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