Yeah, call up the DVLA, same process as declaring a color change.
But you need to make sure what is on the car is on the active. it needs to match or excell the standard spec.
use this guide on parkers to match it up;
http://www.parkers.co.uk/cars/equipment/Summary.aspx?model=232
as for the modded punto. i presume its something on this quoted list...
lol its got a couple of them things but the main one isnt on there for some reason
If you do wanna know i might just post the autotrader link up so you can have a look, never know might not even get this one