You need a new plate and I suspect the official way is to get the manufacturer to supply a new plate. Some manufacturers will have been asked numerous times, others may think this a very odd request.
But since I've had at least 2 trailers that were too old to ever have plates with weights I'm not 100% sure! 1 of them was home made so never had any plate.
NTTA - Trailer Law - Trailer Maximum Weights
My current little box trailer has no plate (almost certainly home built) although the hitch is only rated to 500kg and is stamped as such. That said the wheels are only rated to 395kg I think and I doubt even filled with lead it would weight that much!
By Christmas time it will be more like triggers broom though! Most of it will have been rebuilt!