The only problem is that CG13 are pretty rare in NZ. And all transmission and engine related has to go (that's wat I heard) The GA is a lot easiler to find but cost will probably even out in the end.
To fit the GA16DE you will either have to cut part of your chassis away (near the water pump) or remove the water pump, put a blocker plate in and use an electric water pump.
That isn't much of a big job but I'd highly recommend using the CG platform.. It's a pretty damn robust engine.