if its the same engine the
will be fine with it. If the engine is bigger (do it!!) then it will need remapping, best to get this done by a proper garage but it can be done yourself if you get hold of the correct software and files...
Just dropping a 1.2 engine in should be fine though, remember that your existing
does control other parts of the car (locking, alarms etc etc) and thats why you should keep the same one or lots of programming and stuff will need doing.
You should be fine though...