I stand by my earlier post.
To reuse a prior bread analogy: put a brand new slice of bread in a clean, never used before bag. Just before closing up the bag air tight, put an old (but dry, not toasted, w no evidence of mold) slice at the other end of the bag. Seal it well, ensuring slices never come into contact w each other. Put them in a dark room for a year or 3. Occassionally mist the exterior of the bag w water.
And wait..... see what happens.....
Else, your boat, do as you see fit.
Just because its lasted X yrs done poorly, by the factory or prior owner, is no gauge to how long it'll last left alone, or remodified by you taking foam out.
You'd shouldnt be surprised how shoddy some boats are built when you tear into 1 and expose stuff never intended to ever be seen again once the upper and lower hull went together.
Think of the 10-15yr service life the boat was built for. Your 1981 is nearly 40yrs old, well past its planned obsolescence expiration date.......
Good luck w your project
Ok, I kinda get your point... hence more thoroughly explained... however, what good is foam if it can no longer provide the floatation it once did? What good will dry, crunchy sponges do for the boat, if left in there? Water passes right through it and they get soggy! I see no value whatsoever. And I see only bad news, if you put dry wood on top of them, permitting the moisture they contain to wick into the new plywood.
And using the bread analogy is a little bit flawed. Bread is made with yeast... yeast is a type of mold! Last time I checked, foam and wood are not MADE with yeast. Therefore, they are not intrinsicly prone to going moldy, if given sufficient moisture.. or time, itself. Yes, wet plywood will rot over time with continual exposure, but assuming that exposure is limited and the boat is cared for, I imagine further decay would be very limited.
And I still don't see how the boat is rendered unsafe (or even worse than before) in this approach I'm mentioning. How is it made unsafe? The amount of foam in the boat is minimal, at best, and poorly applied, for sure. And it's worthless now. The boat, if capsized, would sink just as quickly with the existing foam, than without, so how is there any benefit even keeping it in there? At the very least, replacing it with new foam would be a better idea...
Thankfully, the hull/cap are in better shape than the floor was... :-D