A Diesel turbo on a Petrol engine may need the be stripped apart and rebuilt to cope with the higher temperatures. Even then you'd struggle to make the 1.2 to 1.5 bar of boost the Punto should be making.
Depending on what you're trying to achieve and what you have done to the engine so far you can get results from better intercooling, mapping and variable boost controlling before you mess around with bigger turbos.
Make sure you have an engine that can handle the extra boost before you get a bigger turbo. Make sure you can balance the air fuel ratio throughout so you don't detonate the pistons (the way I did putting a Volvo turbo on my Renault 5 GT).
Whatever you do make sure the engine is set up properly on a rolling road by someone who knows what they are doing. D.I.Y driveway stuff is all well and good but never be afraid to ask for advice.