All the D series transmissions (at least 88-00) are interchangeable. Just make sure you have the right flywheel/clutch combination (only an issue for 88-91 setups, not sure in NZ) so that everything mates up.
If you want to put another SOHC head onto your D12, you should be able to use all your stock gaskets. Of course, make sure you get the proper set of gaskets for your intake manifold if you're changing from DPFI to MPFI. If you're going to install a VTEC head, read up on "mini-me" here in the forums and around the net. The process in the US should be much the same as yours.