if a cop pulls you over, they carnt really do anything, its the VOSA that you need to be more aware of. Even if the police check round your car they carnt do you for a differnt engine as they need to be a cerifited automotive inspector.
(my friends a traffic cop) to prove anything.
Iv been pulled over before and had cops check round my car, with a mods, and they have never said anything either.
Although it is advisable to declare everything.
Iv had a few 106's and put differnt engines in them, and never declared it. I atually had a huge accident in one. although wasnt my fault (a van front eneded my car whilst i was parked)
the insurance didnt even check under the bonnet.
but basically unless you do anything major and vosa inspect all your car and details the average traffic cop carnt do you on the side of the road for it, as they do not have any proof.