The HF ecu is OBD0, not OBD1. OBD1 started in 1992 for most Hondas (excluding the NSX). But to answer your questions, no, you're going to have to modify the wiring no matter what. At the very least, you're going to have to buy an OBD0 to OBD1 conversion harness, to be able to plug in the Si ecu. Yes, the Si transmission will bolt up to your engine. However, it depends on what Si transmission you're talking about. If you're talking about the OBD1 '92-'95 Si transmission, then you'll also need to buy a cable to hydro conversion kit. The '92+ Civics have hydraulic transmissions, while the '91 or older Civics have the cable transmissions. They all bolt up together, but it depends more on the car you have.