I don't know anyone who has replaced the stock due to corrosion below the waterline but would suggest that if there is much of it that low then you need to take something very heavy out of the cockpit locker!
My 21 has a similar stock which has suffered some corrosion round the stainless bolts that hold it all together which I have slowed by putting a generous dollop of that yellow stuff round and under them.
The stock comes apart very easily and it should be quite easy to get some Al plate cut to the same shape and anodised if the corrosion is really bad.
Geoff Harwood