A few ideas..
1. The half moon type knobs are fairly common, you could throw out the single one and replace with two identical slightly different but fully fitting / functional others. A visit to a junk yard would be where to start or a used stereo store. This doesn't keep it 100%, but it would work and I doubt even the trained eye would spot the difference. Or better yet, visit a car audio install shop and ask them if they may have something.
2. Send a letter to Sony U.S. with a picture of the boat and the missing knob. Write a short summary of your passion for keeping it stock and how you like to tell a feel good story about how Sony helped you out.
3. I did a search but didn't find much, however, you could look up the other Sony models for that year and cross reference those models in a search... Sony now makes an LED TV with that model number.
4. Search your boat, could be hidden in there somewhere.
Hope that helps, I'll take another snoop around the net see if I can find something.
Edit... I skipped the preset, best bet is to find a used complete unit...that will be tricky.
Edit 2... 5. Keep a eye on others installing new stereos in their boats this winter...they may be throwing out what you're looking for...
Last edited by KahunaCraft; 11-24-2012 at 11:58 PM.