First, the scale of the kit. The box says the boat model is 10 1/2".
The real ones I see on line seem to be 16' long.
16' = 192 ".
192/18 = 10.66" so I'd go with 1/18 scale. Your 1/20 car will look a little small So unless the boat is a part of the diorama 1/20 for the trailer is probably the way to go.
Bill