I have a copy of the factory 'Competition Tune-Up Manual' for the Datsun 1200 & the info comes from there.

The big bore forged pistons are NLA, but by using A14 pistons on A12A rods you can get pretty much the same thing, but more street friendly & affordable. My own 1200 GX engine is currently at 76.5 mm bore size with first oversize A14 pistons. [It was first built in the early 80's & now rests, headless, against the wall in my garage.]
I bored out the little end of my 1200 rods to allow the bigger pin of the A14 pistons to fit & never ever had a problem, but I prefer the A12A rods as they need no mods. The B310 short stroke A13 engine would have the same rods, but would be harder to find as this seems to be an Asian-market-only engine.
If you are going to bore out A12 rods, then get some later ones, say from about 1980 [B310] as these seem to be made from the same forgings as the A12A ones. They look to have more 'meat' around the little end & would be a safer bet.

If you have a spare engine, then this could be the way to go.
If you can't get a big bore head gasket that's made for the 1200 engine, then use an A14 one & simply use a suitable wad punch to make the extra hole that the 1200 needs for rocker oiling. This is what I did the last time I built my 1200 engine & it worked perfectly.

To build your engine to factory racing specs will be both difficult & very costly as just about every factory racing part is now a collector item & is priced as if made of gold.
Personally, I say, stick with the GX induction instead of the factory twin Mikuni Solex race instalation. To quote from my manual, .......

"The SU [Hitachi] twin carburettor, which is standard equipment for the GX, is most suitable for rally use."

A full factory race engine would also not be much fun on the street, as it is designed to be operated at racing speeds, which causes problems with La Systeme' [the Police] as well as being difficult to drive in town.

My big bore 1200 GX was so good, that as a result of owning & driving it, I became the dedicated small-Datsun man that I am today.
If you can get a serviceable 56A five speed, then grab it. It's more fun than an extra 25 hp. Much more fun.