You do need a Engineers certificate for a A15 in a 1200 in NSW. The A14 doesn't need one, this is why the A14 transplant was more common throughout the 80s and 90s. The 120Y would fit under the same criteria which uses percentage increase in the capacity of the engine.

If you have some technical knowhow and tools, doing another transplant will give you better value. But the A15 is almost a "bolt-in" job. But you'd still need a Engineers Certificate in NSW.

Chris