I don't know what plans Chelsea have for replacing Lukaku but Christiano Ronaldo moving to Chelsea seems very unlikely. He wouldn't do that to Manchester United. Lewandoski might be open to a move to Chelsea and with his price tag Chelsea are in a better position to get him than Barcelona at the moment and I think he will be open to the idea of moving to the English premier league.
I agree that Cristiano Ronaldo will probably not move to FC Chelsea. He is still a very loved player at Manchester United and especially the fans of the clubs are really liking him, so i heavily doubt that he would risk to lose all of that positive attitude towards him that was building up over more than a decade with a single transfer.
I also doubt that Lewandowski wants to go to Chelsea to be honest. I recenlty read an interview with the agent of Lewandowski and there he said, that his client was always dreaming to play for Barcelona at least once in his career and now is the chance and that he would not leave Bayern for any other club.