if its like for like then nothing apart from the engine number on the v5
a different engine and the insurance company can ask you for an engineers report - ie they check it over and give it the ok and of course the v5 details (engine number/capacity etc)
i cant think of anything else - it will still need to pass emmissions test at mot time - but again not earth shattering
only other is over car tax on co2 emmisions - but if you dont tell em it will stay as it is - you dont even need to tell them about the engine number as such as its not the main piece of identification unlike the chassis number
insurance will be the biggest issue i would say - thats if they are bothered just depends on their policy on engine changes or what you have done