Pedro, the original manuals sometimes come up for sale on Yahoo Auctions in Japan. If you find one, you can use Moriwaki Man to buy on your behalf and mail out to you.
Can you load up some pics of the NX5?
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30