I just sold my Stuka, having never even flown it. I can tell you that the 91 four stroke would probably be better suited in the stuka though. The stuka will definately handle the extra weight, bigger prop, and greater torque - all of which the four stroke will supply.
Besides, the shoestring may not take the larger prop that would be needed on the four stroke. You might end up with prop clearance issues with the ground. Its actually a pretty small plane. A guy in my club has one and it really moves on a 61 2 stroke. If you mount the 91 2 stroke, it should be extremely fast.