I also expect insurance will be more on a 1.4 modified with a 1.6 engine, then just a standard 1.6 despite them fundamentally being the same car.
I suggest getting a more specific diagnostic of the problem and just fixing the 1.4, do a compression test etc to find out if there actually is anything wrong with your existing engine, it might just be a simple ignition issue or poor fueling/breathing.
In my opinion changing the engine for a 1.6 is simply not worth it.