Having read up about this now, I have found out some interesting things about the Allison engine. It seems that the counter rotation was achieved by engineering the parts so that the crank/cams could be swapped end over end. After that, it was a matter of wiring the spark plugs to change the firing order to change the direction of rotation of the engine. For me, that is quite an interesting solution and alleviates much of the problem associated with keeping two sets of parts.