You certainly have a rare beast there. I don't know of another S30 Fairlady Z with the original L20. Mine had a L20 until a couple of years ago but although it drove fine, it didn't give me the 'wow' factor I was looking for in a Z so I changed it for a L26. I was really torn between originality and drivability but the extra power and smootheness of the bigger engine won although I did want to keep the L-series engine. Also, there were already some non-original parts on it so originality had already gone out the window and I wasn't looking at creating a concours car.
You will find that nearly everything from the front to the dashboard is the same as a 2-seater 260Z but from the dash back is different and you will have difficulty getting hold of parts as there aren't many places that do them for the 2+2 especially panels. The only different part on yours is the carbs. Normally on an L20 they are the 1.5inch round top Hitachi SUs whereas on larger engines they are normally the 1.75 inch although people have been known to put 2" or triple webers or Dellortos on. The 260Z came with flat top carbs on some which, quite frankly, are boat anchors. If you have those, you would probably benefit from changing them, both for power and fuel economy. You might also still have all the emissions gubbins.
Would be good to see some more detailed pics if you have them.
I'm glad I didn't guess at the make of the other car. I knew it was a Toyota but would have guessed at a Corolla GT. Shows how much I know (or not!!) although Mr Tenno had given me a clue which I didn't notice before.