If you lengthen the rod then move the wrist pin up in the piston you are still doing nothing for the stroke, as mentioned above the crank is what determines the stroke. The only way you can stroke a motor without buying a crank is to take it to a machine shop and they can weld material to the back side of the crank were the connecting rod bearings go then they take the same amount off the front side making it have a longer stroke. I don't think this method is used much anymore because of the variety of companies making aftermarket cranks.