Okay, so the way fuel compatibility could work is when a world is started, the mod would scan all valid fluids, and make a list of them in the config.
Then, the user could go into the config and enable or disable liquids being used as fuel, similar to Immersive Vehicles system.

As for the smart car, I'm a bit curious as to why you can't link a key to the smart car.
Is this intended, and if so, how come?

Duplicate request #64
Feature is coming in 1.16 though