If you are going to run a ca motor, they all weigh about the same (much like a series engines are similar, but not identical weights) - so if you have to tolerate an increase in weight, you might as well run all the capacity you can. If 1600cc is the limit, I'd personally be going for a big A series that would end up being around that mark,

I don't know a lot about this engine specifically, or it's revability - but there is such a thing as a ca20 (albeit lower spec, and inferior head/cam ) - you could maybe make your own hybrid running the ca20 bottom end and the ca18 det top end. Or even consider an NA sr20 ( there's a few guys here running them in 2 litre sports sedans category)....

But if 2000 odd cc was off the table, then an A15 I think. If you didn't have to specifically run a nissan motor - the toyota 4ag engines (not sure which exact head/spec you should aim for) which are around 1600cc is worth a look. They can make over 200-220bhp in race trim, from that sort of capacity.

And lastly - if and only if they have a reasonable 'multiplier' for capacity equivalent - you might even take a look at a bridge ported (or peripheral port, if you want to go to the limits) rotary (probably a 12a, as a 13b would almost surely push it over 2 litres capacity with the multiplier, whattever it might be). Interesting thing with the rotaries is they are rev happy, and make power up high (relatively speaking) and you run diff gears to suit such high revving. well they don't produce insane amounts of torque, they just sustain it to a high rpm range - so less torque means less stress on the gearbox. Which isn't a bad thing. They are not without drawbacks, but if I could legally run a 12a in my coupe and get it registered (unless someone knows how it can be done, and be approved, in which case, please PM me!) I'd have a go at it. They are a lot of fun, I suppose a bit like big 2 stroke bikes with relatively peaky powerbands - takes an extra level to drive tem on the edge and keep them on the powerband, but very rewarding when you get into sync with them,