Looking at the front wheel page of the parts book, there isn't a seal pictured, but a 'gasket ring' is listed under the speedo drive gear (part no: 9013171000). I asssume this is the part you're looking for.
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You should be able to get this from Grahams Motorcycles.
The manual this is copied from is available from Mike at
http://www.mzregalia.com/page4.htm for 10 UKP (c.US$20) plus postage.
The headlight fitted to the Sport is of notoriously poor quality, and usually vibrates apart. I've cut one open and epoxied the broken parts back together, but it was only a temporary fix. The best bet is to replace the headlight with one from a Yamaha TRX 850. This bolts straight in, uses one normal type H4 bulb, and is a lot more robust than the standard unit (sourced from a Yamaha FZR 400, I believe). I'm not sure if the TRX 850 was sold in the US; if not, look for a headlight on one of the European versions of Ebay, (but not the UK or Ireland, our headlights dip to the left).
Norman
http://www.mzskorpion.net