Steel diamond-tread (1/8" thick) runs about $350/sheet these days...you sure you want to spend that kind of money??? Plus, you'll need to compensate for the wood thickness versus the plate with more angle between the frame and the plate. Wood deck trailers and steel deck trailers are built differently. When you try to convert a wood deck to a steel deck, what most end with is a bastardized train wreck as a result. Stick with the wood, you'll be glad you did, both for the effort and the end result. Spend the money on some top quality wood planks, and whether they're treated or not, sand them down and finish them similar to how you do a home floor or a patio deck. Repaint the steel on your trailer and rewire, if necessary, first. Then install and finish your new trailer deck. You'll be money ahead in the long run, have a better-quality trailer than what you started with, and have a decking that'll last you as long as ten years, with annual resealing.
Plus, if you haul anything other than cars, a steel floor is a terrible thing to have. Ever drive a farm tractor on a wet steel deck? There's an adventure! Implements tend to slide a bit on steel, too, no matter how they're tied down. Cars will even tend to move a bit on steel. Wood trailers don't have that slippage problem. That's why nearly all of the semi trailers out there have wood decks on them!
I sold trailers for five years, so I have my biases on this.
