I believe balsa to be the best material to use for replacement. If saturated and sealed correctly, it will last longer than it did the first time around. Most manufacturers just did not use enough resin to completely seal all the end grain so when water did get in, it caused rot very quickly. End grain in balsa is situated vertically so that water cannot easily wick through it. Balsa is light, readily available, inexpensive, and proven. West has a good page on doing this. +1 on Rich's statement about it being a nasty job. I found a recip. saw blade or a sheetrock saw is also handy for reaching in between layers and dragging out bits of rotted balsa. While you're at it, you might as well get as much balsa out from under winches, cleats, stantion bases, etc. as you can and replace it with solid glass. Just get some odd pcs. of mat/roving, whatever, wet 'em out and jam in solid under fittings with a long poker.
Alberg 35: With a philosophical flourish Cato throws himself upon his sword; I quietly take to the ship.