CA18's and SR20's are the most common versions of these motors. But you could get a bit cunning and mix and match the various other versions to satisfy the registration requirements.

SR16, SR18's exist.
CA16's also exist. It is just about using the block to satisfy the 'Yes Officer it's a 1600cc' or whatever. I'm not 100% sure that this works, in regards to parts interchangability, but I think it might.

I'd like to see someone do that. You may be able to use the CA18DET or SR20DET head. It would mean getting one of each motor, and a strip down and rebuild. But if you're serious (like many of you A-series addicts) Then this shouldn't faze you.
The CA16DE block / CA18DET internals and head idea is cool, because you can use the CA16DE exhaust cam for more power.

You may also need to look at engine mount points, as the CA16DE, came out in the first Aussie N13 EXAs which are FWD and may have different engine mount bracket holes on the block.
Enough going on. Having a powerful A-series has it's benefits. I saw a fella recently with 74kW at the wheels out of an A15 with a single DCOE weber, in a Datsun 1000 coupe. It went pretty cool. But not even within comparison of Andrew's CA18DET 1200 coupe, especially down the straight at Oran Park..

Chris