It might be worth seeing if MetalMan or Serum can help you out- they are both talented custom builders in their own right- look in Serum's photo album and you'll see the earliest examples of the G2 / G2r specific motor mounts of this type that he worked on along with Mike (there are also threads to match floating around someplace...):
http://picasaweb.google.com/buikpijn/RevoCenterdiff