Car seat and base compatible with mamas and papas MPX pram...????

Hi there, first baby due in 3 weeks!!

we inherited the mamas and papas ultima travel system from a relative with the MPX chassis. 

it came with a peg perego primo viaggio car seat which attaches to the pram, but its a bit old and battered now, so we'd like a new seat/base combo with isofix, which also fits our pram.. 

really struggling to find what I'm looking for. can anyone point us in the right direction?

thankooo