while balsa is certainly, historically, the core
material of choice, one should ask why they want an organic material in proximity to water
decks are always subject to rotting because they 'lots' of holes into the core
using a foam core will provide a higher threshold to the presence of water
(via the aforementioned screw holes), be easier to work with / cut (form) to the size and shape of the replacement and provide better adhesion for the resin.
remember that poly wont adhere to epoxy
so if 105 was there, bite the bullet, pay for the 105 and stick with it. when doing deck
work, i would recommend extra slow (209) hardener which will help with self leveling and better saturation.
if you used foam core you would not need the marine plywood
i am sure u already know that re-coring a deck
is NOT fun. do more research
and make smart decisions and you wont be redoing the core in 5 years.