Rich,

The biggest problem with buying a bike in Japan is that you will require residence here to register it in your name. And you will need a visa in excess of 3 months to get residence documents. It used to be possible to to do "alien registration" with a short-term visa in the past, but Japan revised immigration laws last year. Foreigners are on the same system as Japanese now, and to get on that you require a visa longer than 3 months.
Maybe your friend can keep it in his name and give you permission to use it, but I don't know how that will work internationally. Getting a longer visa may be an option, but they may make you enter Japan's national health insurance scheme if you are an official resident. This will just add up costs for you.
Regarding the Carnet, this assumes that you will return the bike to the country of origin. You won't get your deposit back unless you prove that it is back in Japan or that you have paid all customs duties in the final country. The later involves more paperwork, and the automobile club may not issue the Carnet if you tell them in advance you don't plan to bring it back to Japan. Note that you don't need a carnet for Russia, Mongolia and the Stans.
As for what is available in Japan, see here. Many bikes available in Europe were never sold in Japan.
Japanese Motorcycles GooBike Exchange | Best Place to Find used Japanese and Foreign Motorcycles

Oh, regarding a bike registered in a country other than your nationality: THAT is no problem. I rode my Japanese registered bike with a US passport.

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Japan touring information