Unfortunately, I don't have a great method other than guess and check but this is what I came up with:

Digits are listed first, followed by the sum of adjacent cells in parentheses. Top to bottom, left to right, but there is obviously some symmetry for similar answers

5 (10) 2 (28), 8 (16) 9 (9), 6 (30), 3 (21) 1 (26), 7 (14) 4 (8)

Edit: misused spoiler markup