Honestly build it 3 times bigger, at 30" or larger you can use readily available motors and drives.
Im toying with the idea of adding an Arch to my Boat for mainly rod storage but also maybe radar at a later date. Ya the Pole Dancer has gotten around this year, you know every port I don't notice anything diffrent about the weight, I bet the whole thing only weighs 100lbs or so. New here, left another forum and signed up to this one, just a lot more info and the dually section is a lot busier.
It can be done to get all 4 rear wheels spinning go to you tube and type in armygeddon it can also get all 6 spinning.

That way you can at least get a simulated look of outboard power with the dummy outboard rudder cowling thingy. We had just stopped to get some pictures of a whale breaching, and were headed out for tuna.
I didn't like the little 17" tires so I did some research and bought a set of the Eagle 20x7.5" polished wheels and 275-65-20 Michelin Defender tires.
As far as the outboard goes how about using a outdrive and putting a dummy outboard over it? La Push is in the left background, Hoh Head on the right, and several layers of the Cascades center.

Some dimensions may need to be fudged to fit batteries and servos but that is where the challenge lies.

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