Hyundai Getz - problem with power

Hello,
Im from Poland, sorry for my English.
I came here through a recommendation of my friend.
Having a big problem with my car.

HYUNDAI GETZ TB 1.1 49kw (67km) 2007.

I have been having problems with my car's acceleration for over a year now.
When I press the gas pedal, the car takes a few seconds to gain power.When I'm driving on a smooth road it's fine, just a small hill is enough - then the car loses power, downshifting doesn't help. Checked by 3 mechanics. They don't know what's wrong with it.
So far they have checked and repaired/replaced:

-catalyst
- clutch
- air filter
- fuel filter
- crankshaft, sensor
- HV cables
- ignition coils
- cylinder compression
- reset ECU
- throttle
- injectors
- candles
- oil pressure sensor

The mechanic suggests replacing the engine ;-((
What Do You think ? Some idea's?
Greetings.