You should have cross rails that the wood is screwed to.
You can get deck screws from an 18 wheeler place like Utility Trailer in Houston.
https://www.napaonline.com/en/p/RFDTF120
Get some that are long enough to go through your wood and into the top rail of the cross members.
The cross members are usually C shaped formed sheet metal or angle iron.
You drill a pilot hole and then screw in the deck screw.
Up here we use 2X6 tongue and groove Doug Fir.
Do your odd size boards on the sides.
Rip up the old boards and hit the remainder of the old screws with a hammer to break them off even with the cross rails.
I am going to do mine this spring. Have the boards and Hdw. ready to go.