Just saw this.
You are correct. For a short time, I believe around 1997 the 1.5 in the Festiva had a 67kw donk. From what I remember it was due to a 16v head on the engine.
After that, from 98 onwards the 1.5 engine was an 8v unit that was good for 56kw.
If you are just complaining that the engine struggles up hills this is just part and parcel to driving a low powered car.
I had mine for 10 years - never really worried myself with the power output and just changed down as needed.
If you want to have the higher powered engine I guess the cheapest way to do it is just buy a running car with the better engine.
By the time you look at buying another motor and possibly
ECU and doing the swap it may end up cheaper.
I drove the higher powered car once on a test drive and boy you really do notice the difference!