When I look at the engine crankshaft from the pulley side I see a small protrusion of greater diameter extending from the crankcase seal. You can see the groove towards the end of the crankshaft on the left hand end in the second image. This groove is approximately were the oil seal is. I would investigate the idea of a "beefy cup" area of a propeller extension that would be a press fit over that protrusion and not interfere with the seal. From an engineering standpoint would it be sufficient in transferring the stresses to the bigger diameter or just a decoration?