Nice MOC! The wheel arch parts ARE in LDD -- type in the number 62361 in the LDD Parts Palette's search box (just above where the parts are shown on the left side of the LDD screen). Same thing with the Steering Wheel -- type 30663 in the LDD Parts Palette Search box to find it! You can usually find the proper Part Number in Bricklink, and then "Search" for it in LDD -- it's a lot faster than wading through the LDD Parts Palette trying to find it. Alas, the 11211 "Brick, Modified 1 x 2 with Studs on 1 Side" is indeed not in LDD.
Also, what are the Lego Part Numbers stamped on your black wheels and tires? It's hard to determine the proper sizes by the tiny eBay image you provided. The LDD Part 32146 (combination wheel+tire) works, so maybe the only "missing" part in your LDD model is the 11211 part!