The D15 from a 91 Civic Sedan DX, should swap right into the 91 CRX HF, but is there ECU conflicts or anything? It's all plug and drive right?

 

As long as you use the HF's intake manifold, injectors, and sensors, as well as the HF distributor, you should be able to plug and play. The DX motor is DPFI, while the HF motor is MPFI, so essentially you will be doing a DPFI to MPFI conversion without having to worry about adding the extra wires.