this thread is really good: https://www.beamng.com/threads/tutorial-porting-engines-and-other-parts-inbetween-vehicles.28730/ I would advise starting by modifying existing parts.
https://wiki.beamng.com/Vehicle_Creation_Megapage https://wiki.beamng.com/JBeam_Physics_Theory https://wiki.beamng.com/JBeam_Introduction https://wiki.beamng.com/Introduction_to_Vehicle_Creation If you have any more questions, feel free to ask me.