The 1852 LDS edition accidentally removed the repetition of the in this conjoined phrase (“the power and the miracles of God”). The loss of the repeated the is found sporadically throughout the history of the Book of Mormon text. For this passage, the lack of the repeated the has continued in the LDS text. For another example, plus some discussion, see 1 Nephi 1:1; for a complete list of examples, see conjunctive repetition in volume 3.
Summary: In accord with the earliest textual sources, maintain the definite article the before miracles in 2 Nephi 26:20; this repeated the reflects the original style of the Book of Mormon text.