have you got the old one to re-chrome? Alot of original Jap chrome was really crap, even some of the stuff comming out the factory now. even if you find a NOS sheild it could still have crappy rusty chrome anyway if it hasnt been stored very well.
my first stop for a NOS one would be Yamaha Retro Spares in Adelaide or NW vintage cycle parts or Speed and sport in USA.