Just finishing up a re-core myself - same general area where White Lightning is having problems but on the other side.  You may want to check a couple areas to the the side of the mast as well.  Water got through on my deck where the halyard turning blocks were mounted and it just continued down the slope of the deck.  Core was a black, water saturated oatmeal consistency.

http://www.flickr.com/photos/whiplaunch/sets/72157624530255536/   - this is the project so far.  I came from the top, just couldn't figure how to lay in the core from below without creating a huge mess inside.  If you have time, the cost is really minimal.  Core sheets are cheap.

I'd never done this before but there's plenty of information on the web.  But here's a link to the specific one I read up on and pretty much used as a "template" so to speak.  http://www.westsystem.com/ss/fiberglass-deck-repair-part-1-replacing-damaged-balsa-core/  The people at West and Interlux have been extremely helpful.

Lastly, West System and Sailing Anarchy are great resources.

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High Anxiety #150