The easiest swap is the 1.2 60 engine. This will take power from 54bhp @5500 rpm to 60bhp@5000rpm and torque from 63ftlb@3250 to 75ftlb@2500 rpm.
It's the only "easy" swap tbh. Engine can be picked up for peanuts. Pretty much straight swap but use Cento gearbox, crank sensor, and oil pressure switch. You need to either cut or make a spacer to extend the front pipe slightly for the exhaust to fit but that's pretty much it.
Hi there and a warm welcome to TorqueCars, good to have you with us. I've copied your thread into our Fiat forum as it was a technical question so hopefully you'll get more answers in there.