This would be my first choice. Call Robert and I’m sure he’ll have what you need. They know Chris Crafts better than just about anyone too.
http://www.chris-craft-parts.com/
7650 South Channel Drive - Harsens Island, MI 48028
Robert Henkel, Sales 810-748-3600
Business Office & Fax 810-326-4228
E-Mail scripps@chris-craft-parts.com
If by chance that doesn’t work, then I’m sure Marine Parts has this common thermostat. I say common, because I’m sure there are still a lot of these motors still out there using this same part. I don’t see it specifically listed in the Marine Parts web site, but I am quite sure all it will take is a telephone call to them.
561-748-0053
http://www.marineparts.com/sierra/sierra539.aspx
Hope this helps,
Tom