I also have Marina disc brakes on the front of my Minor and a good set up it is too.
As regards fitting the Marina disc set up to a Austin A40 - the Marina mounting arrangements are completely different to the A40.
I would suggest that the MG Midget/Sprite set up is the easiest to fit and maintain. Near enough a bolt on conversion with no faffing around and having to make and modify parts.
If MG Midget/Sprite parts are in short supply in NZ you can always purchase in the UK and ship to NZ.
For example - I have a BMW motorcycle twin cam head and fuel injection set up awaiting shipping to Queensland, Australia which is going to be fitted to a classic Mini.
Here is a Midget/Sprite disc brake set up suitable for fitting to a A40 :-
https://www.ebay.co.uk/itm/233557925477 ... Sw4HFelDU1
If you have a contact in the UK who can hold the disk brake assys whilst you organise the shipping would be of great help. That is what is happening with the BMW items. I am just holding them whilst the shipping is being organised.
The MG Centre Wrexham (see link above) may be able to arrange shipping for you.