The 1.6 engine is a derivative of the 1.3 and 1.5 engines originally fitted to the X1/9
The other petrol engines in the Punto (ignoring the 1.4 turbo which is again a derivative) are not straight swaps and belong to a separate family of engines (FIRE). As already mentioned they can be made to fit but it isn't just a case of bolting on the X1/9 engine mounts and after you've worked that out there is the matter of the gearbox as well...