Personally, I don't think it's a very good idea to mount a nuvi on a motorbike as I think the shock and vibration with shorten its life. But if you want to only use it occasionally, probably if you buy on of these kits:
RAM Garmin nüvi® 710, nüvi® 750, nüvi® 760, nüvi® 770 & nüvi® 780 GPS Mounts
and couple it with a standard ram handlebar mount for a motorcycle, that should do the trick.
HTH,
David