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IIRC - 1986 Federal Law prohibited 2-stroke street vehicles over 49cc. The '84-'85 RZ 350's marked the official end of the 2-stroke street bike in the US. It is still possible to circumvent Federal law, and get State approval, though, but the importing of any bike over 49cc into the US pretty much stopped in 1986.
The last "Legal" 2-stroke may have been the Bimnota V-Due - which was made in 1997. I suspect that Bimota must have had to jump through some serious hoopage with that one. Wiki indicates that this was, indeed, an emissions issue. Ironically, it has been dubbed "The bike that killed Bimota": http://www.odd-bike.com/2012/11/bimo...ed-bimota.html