Technially no its not legal, if the new shell you have was unstamped you would be able to transfer the idenitification over from the K reg one but you cannot remove 1 set of identification and replace it with some from another it would be classed as a ringer.
But seeing as you own both cars and they are not stolen personally I would find somebody very handy with a welder to put the numbers in so you cannot tell if you really want it on a K plate lol