^^ show us photos, good opportunity to learn.
I was figuring it would not be curved for the say 40 cm right under the mast step, or at least not curved enough that it could not be made flat with just a very little thickened epoxy
above the G10. Then scarf that into a curved wooden beam (probably laminated as you suggest, but would depend on how much curve is actually there which i can not tell from the photo) that fills the gap he has cut and scarfes into the ends of the beam that are still in place. (
So: old beam port side, scarfe, new curved wooden beam, scarf, G10 under mast step, scarf, new curved wooden beam, scarf, old beam stb side.
But I will be delighted to see and learn from how you do it. And I agree a single
laminated piece will be way faster and cheaper than what I am suggesting, and probably last longer than the owner.
This is all much easier on an aluminum hull