DON'T USE EXPANDING FOAM UNDER ANY CIRCUMSTANCES!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Expanding foam will continue expanding every time it gets warm and will, eventually, blow the boat apart. I would get some of the blue insulation foam, cut it into blocks and drop it in between the frames. Now, for your other things you received:
- The rudder bracket could be an issue, due to it's length. It will make the boat act like it's longer than it really is
- Servo mount would be fine if you install it under the cowl and point the arm down. What servo are you going to use for the rudder?
- Do you really need that big of motor? That looks like one that would be used in an electric 1/8th scale