Mark 1 what?,
I'll assume its a Punto if so I think the general consensus is that you can safely fit wheels up to 16inch, People have put bigger wheels on but at the expense of rubbing,tramlining etc.
As always it depends on the wheels fitted such as offset & width.
Try to stick as close to the original fiat requirements regarding Offset and PCD and you should not have any problems.
Hope that helps.