Leaving an old weight bench outside for about a year, the wood had dry rotted. My oldest son wanted to start using it, but the would could easily be broken up in your hand. Since there was no major rust or structural damage and I was gotten some free scrap "composite-plate" from work, I decide to repair the bench instead of replacing it.
Step 1: Replacing the the wood parts
Using what was left of the wood as a template, my son and I cut out the shapes with a jig saw. Salvaging the hardware and marking where the holes needed to be drilled.
Step 2: Test fit the pieces
Drill the holes for the hardware and test fit the pieces.
Step 3: Add padding
The replacement padding ended up being knee boards used for water sports. They are thick, easy to cut, and are not supposed to grow mold. Once the pieces were cut we used spray adhesive to keep them in place.
Step 4: Re-vinyl
Using the spray adhesive to tack down the vinyl to the padding and then secure it. Since the composite place is metal and plastic we used self tapping screws into plumbers tape to secure the vinyl.
Step 5: Test it out
The thicker padding keeps your shoulders from digging into the backrest. Bonus.