Right, you can't modify carb bodies made with cable pull actuation for a shaft-and-bellcrank system. What signifies in carb size is the unobstructed area of the venturi (throat), and a 33 mm. straight-through carburetor provides nearly as much unobstructed area as a stock BS38. Although no longer available new, the Mikuni VM33 is a bell crank carburetor that was original equipment on a number of "superbikes" in the late '70s and '80s, and good used samples are fairly common. You'd need to fabricate brackets and a throttle shaft for the XS650's 120 mm. carb spacing and do some jetting, but since the VM33 was set up only for 4-stroke applications the jetting task should be fairly straightforward.