Finally have time to start this rebuild, would be nice to hear some advice how to do this correctly. Plywood type? Epoxy? Polyester? Shape off new stringers? Need strenght for 70mph speed

Are you sticking with I/O setup?

Before removing much more structure make sure the hull is supported uniformly so it does not distort. I use vinylester and plywood, and I use epoxy for fastener reinforcing.

bbc 509cid

why is vinylester better than polyester?

Much stronger. Almost as strong as epoxy. Bonds well to existing laminate and to plywood.

Vinyl ester is also water and chemical resistant
downside is limited shelf life

I guess i am going with polyester, even the vinylester is 3 times the price off polyester here in Finland. Local fibreglass guy has mold for U-beam stringer, plan is to make the stringers from U-beam, twice as thick as the small sample in the picture.

Interesting with the U beams, just make sure you build up a layer below the U to spread the load as much as possible.
On a conventional type build, I would use 2 layers of 3/4" plywood for a structural location, giving you and area of 18 square inches (plus fillets) per lineal foot to spread the load.
The U looks to be about a 1/4" giving you 6 square inches per lineal foot, and only fillet on one side of each vertical. You mention twice as thick, so the layup, width or height?
Just something to ponder, unless you have addressed it already.
Start a restoration thread!
The 260 Stinger is an awesome hull, and I'm happy to see you putting the effort into one

We make the same U-profile as in the picture but double the material thicknes. Have heard that this litle 26 is good in rough water for its size. Going to try high X, 17-18" for 4 blade props, no notch.