Locked diffs are illegal and dangerous on the open road in NZ and Hastings doesn't need any more boy racer bad press and tragedy: Too young.

Do you know why the A12 is running on 3 cylinders? You may find it is a simple fix and an A 12 that is running well is a good match for the 1200.
The best engine for a power upgrade for you is an A15 out of a ZX Sunny. Forget the 5 speed as you are going to blow it with the way you plan to drive. It's cheaper to replace the 4 speed as needed.