Without talking to your local code department first, the 1" x 6" plank boards offer additional rigidity to the structure, even though the rafter spacing is unorthodox and seemingly too wide for current code requirements.
If the remaining 1" x 6" plank boards, after you do the replacement required of defective ones, are not significantly deflected or pose a structural issue, and do not have a 1" to 2" gap in between each board, you should be able to apply the new shingles directly on top of the existing dimensional deck boards, without any concerns.
But check with your code department first, if that is acceptable, which it should be.
Ed
If the remaining 1" x 6" plank boards, after you do the replacement required of defective ones, are not significantly deflected or pose a structural issue, and do not have a 1" to 2" gap in between each board, you should be able to apply the new shingles directly on top of the existing dimensional deck boards, without any concerns.
But check with your code department first, if that is acceptable, which it should be.
Ed
