Isaiah 12:6 (King James Bible) cry out and shout thou inhabitant of Zion
In the New Testament, the King James Bible uses the alternative (now archaic) spelling Sion for Zion. Except for one use of Sion in Psalm 65:1, the King James Old Testament has only the spelling Zion. As shown here in 2 Nephi 22:6, the New Testament spelling Sion intruded into an Isaiah quotation in the Book of Mormon. Originally introduced in the 1852 LDS edition, this spelling was maintained in this verse in the LDS text until the 1920 edition (with the exception of the 1906 large-print edition, which had the correct Zion).
The alternative spelling Sion is found nowhere else in any of the Book of Mormon editions or the manuscripts. There are 44 other occurrences of this name in the text, and all are consistently spelled as Zion.
Summary: Maintain the current spelling Zion rather than the archaic Sion throughout the Book of Mormon text.