The Beetle engine and drivetrain crate can be found in between a hairpin corner along Interstate 620 in Mount Providence.
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 Mac: Shift-Lock questline in chapter 3 - City Lights and collect the Beetle chassis, before they can collect the engine & drivetrain.
Collecting the engine and drivetrain crate will deliver it to Rav's garage on Airfield 73.