Get that useless foam out of there, you can't even work the way it is. If you want to put back, your choice, its there for flotation, but at this age its probably saturated anyway. That kind of foam is like a sponge.
Use cardboard to make a template, then cut out the wood. Don't use plywood for stringers. Use a good quality piece of real wood. Were not talking a lot of wood, so for me I would not be picking out some cheap/wet treated 2x4. If you just have to go to the HD, then at least pick out a piece of oak or something. One continuous board.
You will need to rough up the glass where you are trying to bond to the hull well. Needs to be nice and clean too. There are different theories on how to let that board meet up with the hull. Essentially you want a good bond but don't want to create a hard spot. The easiest is just to use some peanut butter consistency resin to gap fill and spead the load around the edge of the stringer as you lay it down. This has the advantage of being material ready to then bond with your first layers of glass. You will also want to fillet the edges real nice where the stringer meets the hull. You could mess with something more flexible like 5200 as the bedding point. There are advantages, but honestly few go to this length. Up to you. Forget the gorilla glue. Get the stinger in place, filleted nicely on either side, then lay the cloth over it and start wetting it out to encapsulate. Take a squeegie after wetted out and get all the excess resin out of it. Don't try to reuse, just scrape the excess out into a pail. Let this layer set up until it gel's, then before it hardens, use a razor blade to remove the excess cloth. If you cut at the right cure stage, it will come out nicely and clean. If you go too soon or too late, or try to force it, well, you will learn why you wait till the right moment. Layer on at least two more layers of cloth on top of this and your pretty good to go. If you have to cut limber holes with a hole saw, make sure you seal them up with resin so you never have to do this again.
The easy way to dispense the thickened resin mix (peanut butter consistency) is with the empty caulk tubes. You can find them at west marine for just a few bucks. they are disposable. You can thicken with wood flour (thin sawdust), wheat flour (really), or cabosil depending on what is available to you and your budget. Epoxy is best, but even with inexpensive polyester resin, this job done right will never need to be done again.
Have fun.
