I would search this on the G40 forum. Looks like the 1.3 maybe used but with tapping and modifying which may not be worthwhile in the end. So the block is different, pretty sure the head is too.
everything[pistons/crank/cam/rods] is different from the normal 1.3 [mh/aav/nz etc]even the head,it has squared inletports instead of round ones on normal 'smallblocks'.
Head casting is the same. Exhaust valves are different to the normally aspirated 1272cc heads though. I mapped an AAV (1272cc 8v n/a) motor with a K03 turbo on it using G40 management. The owner had done 20k+ in it last time I heard at around 12psi of boost, so the standard blocks are relatively tough too.
