There is only one legal number plate for your motorcycle and that is the one that has been issued. A number of years ago they used a smaller plate and it may be seen on older bikes. Just like the USA, the plate they give you is the only legal one.
I got a smaller one made for mine. Not much smaller - just a fraction and so far I have had no problems with the police. I would imagine though that if it was too obvious you would called pulled over quite often.