If you just pop it in and use the 1.2 ECU and wiring, fuel and ignition will be fired as the 1.2, restricting power and possibly melting pistons if the fuelling and timing do not suit at all.
Ideally you'd need the engine ECU for the 1.4, but then you may have difficulty getting it to work in the "wrong" car. Initially the engine ECu and car body computer will refuse to communicate. A proxy alignment using
MultiECUScan might work, but there is a high risk of it not doing so.
Engine, engine computer and body computer from a similar model should work, in theory, but would need identical options, such as foglights, airbags, aircon, etc., otherwise "new" body computer will then not operate the car properly.
Once you get it working, your insurance company will treat it as a special and hike the premium.
Save up for a complete car might be a better idea. Insurance will treat it better too.
If funds are insufficient, relax, go with the flow and enjoy the 1.2.