Have you visited www.400gb.com? They have many many guys on their forum who run Jap only models, and will probably be able to help.

Personally I'd try Elliot motorcycles in Cricklade as they import stuff themselves, and have been quite useful in sourcing NSR bits.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball