Good to see another person getting intrested in S12s. There are a large range of engines available depending on where you are in the world. It the uk they mainly came with a CA18ET. In the states there was a choice of
CA20E, CA18ET or V6 VG30E. And in the Japan and ozz they came with FJ20E & FJ20ET aswell. I believe few late models did come with CA18DET. 2 body styles available fastback or hatch, hatch is more commonly known a Gazzelle. As for tuning and part or lack of parts most of it is fabrication or making s13/14 bits fit, such as coilovers.
I think thats about it hope this helps and any questions you have someone one on here will be able to help you with.
Do the right thing and buy an S12
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Welcome again