d16z6 and d16y8
The d16z6 and d16y8 amount to the same gains. Both are economical single-cam true-vtec engines. Both are common in salvage yards, available in 1992–2000 Civic EX and Si’s, and some models of Del Sol. Power output is roughly 130hp for each model, but a frankenstein of the two is reputed to work best. The d16z6 has a better-flowing head and the d16y8 has a better-flowing intake manifold, so putting the two together on top of any d16 block will yield some power gains.
The most notable difference between the two is the onboard diagnostics. The d16z6 is an obd1 engine, while the d16y8 is an obd2 engine. Both can be converted to obd0 easily enough, or your wiring harness can be converted to obd1 (obd2 is not recommended for engine tuning other than specific applications). During the wiring harness conversion, the vtec solenoid should be wired into the ECU (automatic ECUs are particularly useful for this modification).
Availability of parts and relative ease of engine installation make both these engines excellent candidates for any CRX.