The Z250 underwent a few changes of cylinder head and barrel while it was in its 'A' state. the biggest change as between the A1/A2 machines and the A3 machines - although the differences are largely to do with frame and equipment.
It seems that there was an important redesign of the cylinder head at the end of the A2 models that ran into the A3 and subsequent models - from what I can gather these changes were concerned mostly with cam bearings, camchain tensioner system and tappet covers - but anecdotal evidence suggests that the cylinders and heads aren't intercahngeable.
a part number chronology:
engine number E00101 to E007343 = part number 11002-5002 - superceeded by 11002-5009 then 11002-5020 all those parts are interchangeable.
engine number E007344 to E016388 = part number 11002-5008 - superceeded by 11002-5020
engine number E016389 to E017142 = part number 11002-5019
engine number E017143 to E017569 = part number 11002-5008 - superceeded by 11002-5020
engine number E017570 onwards = part number 11002-5019 - superceeded by 11002-5020
this info has been got from the parts catalogue for A1/A2/A3 models and I am not sure how accurate it is - especialy because the numbers seem to overlap.
this information implies that late A2 machines had slightly different engines than earlier A2 machines.