The Bel Air engine and drivetrain crate can be found near a cliff edge in the northwest region of a town in Silver Canyon.
Nearing the part will trigger a sonar beep, and will increase in rate as the player nears the part. Collecting the part, once prompted, will deliver it to Rav's garage on Airfield 73.
The player is required to complete the Jess: Runner questline in chapter 3 - City Lights and collect the Bel Air chassis, before they can collect the engine & drivetrain.
Collecting the engine and drivetrain crate will deliver it to Rav's garage on Airfield 73.