Hi, interesting diorama subject. What scale will this model be? I saw a diorama of Seabees in Vietnam in a museum a few years ago. It looked to be 1/72 scale and the modeller scratchbuilt an, "A" boom with a front bumper winch, attached to the front of a GMC truck. I guess, try railroading models. Construction equipment is not super popular so it's hard to find kits to replicate. If your model will be 1/35, try the 1/35 - 1/32 scale diecast toy lines available online and at some toy and gift shops.
hope this helps. keep this thread posted with progress.