Hi Chopper, The OS .91 is good engine, but in the Viojett it tends to be made out of glass. The reason that it will not fit in the Viofan is that the carb is offset in the backplate, the same reason that you cannot fit the engine fairing on the back of the Viojett. You might try seeing if you can pick a good used BVM .91 or .96, there seems to be a good quantity for sale right now. That inflight mixture is an engine saver. Bill Harris