A good start is either www.datsun1200.com
(a mostly Australian forum full of 1200 lovers) or www.ratsun.net
(mostly people from the northwest US but they have a somewhat active collective of 1200 owners. And of course there's here.
Messing with the stock A12 engine is hardly worth the time. With minimal fuss you can swap in the newer A14 or A15 from a B210/210. The A14/A15 swap alone is a big improvement, and the A14/A15 will respond better to "massaging" than the A12 will.
With a little more
fuss, you can swap in an L16/L18 and still look fairly stock and have quite a bit more fun.
With a lot more fuss you can swap in a CA18DET (1.8L turbo) or SR20DET (2.0L turbo).
All have been done many times into 1200s, so the research is already done, and there are people who can help if you get stuck.