Yep, you've lost the thrust washer that's supposed to fit between the prop driver and the inner race of the front bearing. Going by the parts diagram at http://61.221.10.43/english/products/eng/9130.htm#90071 you'll see it's listed as part #12 which seems to be part of the gasket set. This washer is needed because your engine doesn't use the tapered collet drive but flats machined onto the crankshaft. I wouldn't get too worried about it though because any steel washer that's a close fit on the crankshaft but smaller than the outer race of the front bearing will be fine. All it's used for is to transfer the tightening force on the prop nut through to the inner bearing race. With a tapered collet drive, the end face of the collet presses against the inner race to do the same job.