Tony:
In the "Hull and Deck" section of this site you'll find two notes I've posted titled "soft deck" and "installing deck hardware".
The work may seem a handful but it's actually very straightforward and you'll be rewarded with a really solid deck and no
leaks from either hardware or conduits. I'm my view, you don't need to replace any core material--that would be major surgery.
If you have any questions regarding all this, please don't hesitate to contact me.
Nicholas
P.S. Anytime I feel like I'm spending a bit too much time and effort on my Ballad, I remind myself that they don't make boats
like this anymore--at least not in this length and price range. A Ballad is really worth keeping.