what screws for rotted step

Use a few nails of any reasonable size, but anchor the new piece down with generously applied construction adhesive. The end result will be much firmer underfoot and perhaps a bit safer. Best to do it when everything is thoroughly dry.

Joe

Nothing more permanent than a temporary repair. Don't make it too pretty, or you will find reasons to put off the rebuild till the snow flies. Plywood is the wrong material, BTW- narrow strips break easily when they get wet a few times. Use a couple pieces of 2x4 or 2x6, or even 5/4 deck boards. For a truly temporary repair, I'd just use flooring/siding nails of an appropriate length- no point in making the demo any harder than it has to be.

If you don't feel like relearning how to lay out stringers, the Borg deck aisle has a nice selection of precut stringers and treads that make the rebuild job painless. BTW, measure the height and depth of each step before you screw it all together - they make some assumptions about how it will be installed that may not apply in your case, and you may need to clip 1.5" off one face, or add a sleeper at the bottom. DAMHIKT.