If you want to run a vvc inlet manifold on a 1.4 then the ports in the head need to be machined as they don't line up correctly. Even then it doesn't really give any gains on a 1.4. Some people even argue it hinders performance.
As for running nitrous I'm afraid I can't really help with that.
As for the bike carbs, again as you said it would be complicated, and again I'm not sure if you would see any significant gains for the cost involved.
If you want a power increase as they say there is no replacement for displacement, so I'd be looking at either a 1.8 or vvc engine or even a whole car as they are getting cheap these days.
If I was you I'd prob try the nos on a standard setup and if it goes bang, then think about a bigger engine.
Keep us posted.