If I recall correctly, the Cinq Sporting has the same 1108cc engine as the Punto 55. So the 'box off a 55 should fit (although the ratios may be slightly different), and the one off a 1242cc Punto 60 probably will too (as its near enough the same engine). But the 60 'box has higher gear ratios.
The lesser Cinqs use the 899cc engine though (based on the old 903cc OHV unit) and I don't think the gearboxes are compatible.
It's well worth asking in the cinquecento section on www.fiatforum.com too, there's loads of Cinq owners on there who know their stuff.