Stringer size

I am not sure, but I think the stringers are more than 1.5" thick. I have heard that people simply double up 2x lumber. This practice should be fine in theory so long as you use plenty of mechanical fasteners to laminate the two together. I also would suggest you epoxy the two pieces together as well. Use a thin epoxy mixture initially (to soak into the grain) followed by a thicker epoxy (gap filling) before joining the pieces. Then use stainless screws in an altenating pattern to hold the pieces together as the epoxy cures. You can remove some of the fasteners if they get in the way of your final shape.

Now if the overall thickness needs to be less than 3" then you can have the assembly planned down to proper thickness after the epoxy has cured and the screws removed. Install new shorter screws after it is planned down. At this stage, I would epoxy (thin) the screws in place to seal against water instrusion into the grain of the wood via the screw holes.

Finally, before installing the stringers, I would thouroughly coat the entire stringer (after cut to final shape) with thin epoxy to seal the wood against water instrusion. Pay particular attention to end grain as it will soak up lots of epoxy. Now, before you glass the new stringer in place, be sure to wipe in down thourougly with acetone and then use a course paper and sand the entire stringer to give the epoxy coating tooth so the epoxy resin will stick to the stringer when you glass it in place.

Have you noticed I never used the word Polyester? Only use epoxy resins when making repairs...it will make the strongest possible repair and is easiest to mix correctly for proper curing. Epoxy resins can be thinned with Acetone up to 20% by volume. Sawdust, Cabasil, talc and other fine powders can be used to thicken epoxy resins.

Thanks, I have found 2x10 fir here in Orlando. It may be a little large but it gives me room for error and the bottom of the old stringers are a great template.I will be using epoxy (West System)
resin for the project. Now it's back to the grind(er).