Can you re-make a method abstract in the inheritance tree?

Question

EDIT:

To be clear: The fact that the design is quite ugly is not the point. The point is, that the design is there and I am in the situation to have to add another sub-c

Answer 1

You want children of MotorizedVehicle to have a default implementation of bar, but not so for the children of the FlyingMotorizedVehicle.

abstract class BasicMotorizedVehicle
    // no bar
    ... // Rest of old MotorizedVehicle

class MotorizedVehicle extends BasicMotorizedVehicle
    Foo bar(...) { ... }

class FlyingMotorizedVehicle extends BasicMotorizedVehicle