the easiest swap would be the Yikes G4, which is practically identical to the B&W except for a few minor changes. The next up would be the Sawtooth, but as noted by capuchin, you would need to do some dremeling work among other things....
Just remember, the further up the G4 line you get, the further away from the B&W design you get as far as mobo layout, backplane changes ect