G'day mate and welcome to the forums,
To tell what engine you have, you need to locate the Engine ID Mark which is on the engine block itself. The number is on the left (distributor) side of the engine.
I am led to believe the 120y's came out with A12, A14 and some even A15.
There are variations of each engine.
If it is in fact an A12, there was an "early" and "late" version. The later version (1974 onwards) will have the distributor toward the middle of the block as opposed to being right up the front.
Visit
Tech Wiki - Nissan A Engine for more info
and
Tech Wiki - A12 Engine.
Tech Wiki - Engine ID MarkDatsun1200.com member "bige" can help you with a full rebuild kit as can several other places including precision international.
Hope this helps.