This MIGHT help
http://cgi.ebay.co.uk/SPEEDOMETER-CONVERSION-KIT-SPEEDO-STICKERS-MPH-or-KPH_W0QQitemZ300339391431QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item45ed9f6bc7&_trksid=p3286.c0.m14Electronic version for jap bikes..
http://www.busters-accessories.co.uk/productinfo/CKBY01/Tuning/Speedo-Converters/MotraxBut is your speedo cable fed or wires? I.E. what year-model is it?
When I put my old 87 KTM350 on the road I got a mountain bike speedo from Halfords..works with magnet on the spokes and calculates using wheel height.. passed the first registration test and three MOT's...