As above, DO NOT change the number on your VIN plate/floor/V5

BUT YES advise DVLA of an engine number change....

Though, you will find the engone you are going to fit does not have a number, as It was a brand new unstamped short engine from Ford... Up to you what you do with it, leave blank or stamp with yer own number

Personally I would probably leave it blank that way if plod do get funny, they can straight away see it hasnt been tampered with