Anything is possible. Forget the conversion kits, all you need to do is make a motor mount, find place for the battery, and get a pitch 1 pinion gear. That's probably not all of it, but those are the basics. Now my only concern is, are you going to be using the metal chassis? The reason electrics have plastic or CF chassis are for the weight, and I would think that leaving the metal chassis would stress the motor. Other than that, this should get you going.
BTW: They did once sell an electric RS4. I had one, but no part support (Discontinued).