Agreed, it is definitely made in NZ. Try looking
HERE..., as this may give you info. on the existing engine. There are also links below on the page, for further reference to it.
This is probably a long shot, but can you tell us what the existing serial number of the mower itself is? [your photos show it but the part of the # that is visible, I can not read] and we can possibly work from there...
Cheers,
mice_elf