hi will a punto 75 mpi fuel pump have enough guts for a punto 85 1.2 16v engine,i think the p75 pump was 2.6bar but ive no idea on the p85 one?

Can't give a definitive answer: the P85 one will be 3 bar (nominally) but that only covers the delivery pressure, not the delivery rate.

Logically you'd expect the larger engine to require more fuel.

To my mind, second hand fuel pumps don't make a lot of sense (even in N/A engines) when you can buy a new insert for £70 or so.

Are you asking about the pump? or the fuel pressure regulator?
Pretty sure the 60,75,85 all used the same pump (Bosch 0580313026) -but could have had different pressure regulators.
Prob best to read some of the many threads by people who have done a 16v conversion (Todger, arc, Dan etc, etc, etc). Think most of them used higher flow stuff like the Walbro255LPH.