believe it or not you used to legally be able to swop ids on vehicles if enough parts of the donor vehicle was used it was back in the days when cut n shuts were less frowned upon if done properly it came about when if you were using the rear of one car and front of another and both ends had vin numbers on they obviously would not match so you could use the vin number of the other vehicle for both ends, nowadays it will get q plated tho and the chassis number has to be removed entirely.

also changing the chassis number by cutting and welding the number in isnt as easy as it sounds to get away with, have you ever seen the police xray a chassis number? i have and the weld no matter how well ground shows up like a sore thumb!

i dont see why you had trouble changing your engine from 1.6 to 2 litre, when i did my escort van i changed engine, fuel type, colour and overall weight just filled in the appropriate parts of the logbook sent it away and recieved it back changed with a letter thanking me for doing so and some info on my van now being in a different taxation class