well.......
I have a atlas Classic RS-1 in SP&S black Scheme, and as far as I know, their is no drop in method for the decoder instlation, but IIRC, I think the motor is isolated from the frame by a plastic cradle, But I am not sure, overall I do remember that it will require either removing one of the lightboards to make room for the decoder, or milling out a spot for the decoder.