first of all, if you have an accident now before selling it your insurance company will declare that because the engine is not as stated on the log book and you have not declared this to them your insurance is invalid. even though the engine has been down graded from a 3.1 to a 2.8 in their eyes it has been modified. if you wish to sell it as it is i would inform the prospective new keeper of the situation as it might come back to haunt you later. i would get him to sign a piece of paper acknowledging the change and let him take the appropriate action.