That 3-layer foam referenced above from Defender is like one of the stock noise-reduction products offered at our local heavy equipment vendor. I sometimes still wonder if I should bought it rather than the lighter-weight single layer that I did buy. The factory sound deadener was a single layer with the shiny mylar surface adhered to it. In either case, the engine noise now is
greatly reduced -- how much of that is from the new Betamarine being inherently quieter, I do not know. I DO like the result, though!
Actually the original stuff was probably doing OK, except that after the first 20 years the un-sealed edges had allowed deterioration of the foam product, working inwards until most of the old foam was falling off. I had removed the old stuff many years before. (Gawd-Awful mess) Given that the product does break down, slowly, in the presence of heat as well as fumes, replacement after X number of decades might be needed anyway.
After talking to my boat builder friends, I sealed ALL of the sides/edges with a ($) tape before gluing the new foam panel pieces. Lastly, following their advice, I added short screws with large alum. washers, as a sort of "belt & suspenders" approach. You can see this in my engine replacement blog here.