As I posted earlier in this thread the t-stat gasket is not unique to the marine engine and is a two dollar part at Advance Auto. Same for the thermostat, 170 degree, also available at Advance Auto. I am using a piece of aluminum tube for the sleeve. Since it is not stepped like the one from Toyota, there is a small ridge inside the t-stat housing that needs to be ground off (very easy). Once I get my home brew system done I will post all the parts I used in one list with a short writeup. And if you want a piece of aluminum tube (I have a 3 foot section) let me know. But wait till I test the setup, that should be within the next 2 weeks or so.
Of course, since I have no installation constraints (i.e. it is going in a boat with alot of room around the engine), my installation may not be friendly to installation in your ski boat, YMMV. However, once you understand how the fluids flow it is not hard to configure your own system. And some of the parts I am using could be can no matter where you install the heat exchanger.