Change one engine for another is it legal?

Hello everyone, my question is in Spain. I know that if you refrain the engine of a car you can go to scrapping and put the same CV engine and displacement, fix the traffic and solved papers.
My question could you put one of Mallor soured or minor being the same manufacturer? (Example: A4-2.2 Put 1.8 of the A4 Audi)

and 2nd could a engine of different manufacturer and different displacement be put? going through tuning omologation and others. (Example: Aston Martin DB7 with Audi 1.6 engine).

I ask this why I would like to buy a large classic car for weddings and have the reliability of a new engine and consumption.

You can change the engine you need a role that is a workshop that is responsible for that it is well mounted and everything and then you need an engineering project where you usually change these things like cushioned brakes ... doing can be done but that is expensive that if you want to mount the one of another brand.

If it is the same PJ brand a SEAT and it is the same model and all the same less displace