It is normal practice nowadays for the airframr manufacturer to have the engine manufacturer to supply the power plant package to hang on the wing, including the nacelle. Differences in engine design will mean that the cowling and inspection hatches will be different for for a IAE or GE powered aircraft. All engine related checks are regulated by the power plant supplier, which lets Airbus off the hook in case of some failure. 40 years ago when I was flogging aircraft, things were lot simpler, no engine choice and the customer only got to choose the paint job!!!