https://www.beamng.com/techdemo It only has a Covet on Gridmap, and no parts selector, so just finding an older version of the full game would be more fun, but there you go.
If you own the game via Steam, you can go into the properties, then betas, and then you can find 0.4.0.6.
I'm sure there are people out there with the original techdemo. It probably wouldn't be too hard to find someone who would give it to you. In the meantime you can get older versions through steam.