No matter what engine setup you get, you should make sure that you get an engine/tranny combo that includes a CABLE operated tranny for the clutch operation, and a factory LSD would be a very nice touch as well.

The B16a1 is a JDM engine, 1.6L DOHC VTEC rated at either 160 or 170HP depending on specifics. This is a swap favorite because it fits fairly easily into a CRX [it was factory on some JDM cars, so there ya go...], and due to the numbers in which it was made, is readily available very cheap.

The ZC is another JDM engine, and can be found in either a 1.5L SOHC VTEC configuration or a 1.6L DOHC non-VTEC, both of which make around 130 HP.

B18's can come in several varieties, all of which are DOHC and 1.8L in displacement. The B18a/B18b engines, commonly referred to as an LS engine [even though they were also found in Integra RS's, and GS's] are non-vtec engines with good torque, about 140ish HP stock, and a fairly low compression ratio which makes for a good turbo candidate. The B18c is can be found in the newer GSR's, DOHC VTEC, makes a bit more power than the SiR version B16, and has more torque. The B17 engine was also DOHC VTEC, 1.7L and is basically a sister-engine to the B16. The B18C5 came from the Integra Type R's, 190ish HP stock, high revving, high compression.

The main concern with B18's in a CRX is block height, since this was a major concern a few years ago, and the head could hit the hood. Place racing and HASport mounts have pretty much eliminated this concern, just be aware that there could be such a problem. Beyond that the only other major problem is that the factory intake manifold on a B18c will not fit in the engine bay, it will hit the firewall, the LS, and C5 don't have this problem.

If it were up to me, I would go with a B series engine over a ZC for the simple fact that there are lots of transmission and gearing options available without sorting through the relatively undocumented area of D-series transmissions.