As far as I know - which you'll probably see on K20.org x100 times....
Motor
Trans
RSX-S shifter and linkage
K-Pro, Hondata Reflash, or JDM ECU
Custom Axles made, or Hasport Axles
Hasport Mounts
Relocated Radiator/hoses made to fit, or an RSX radiator made to fit.
Coolant temp/thermostat insert for hoses
Fuel reg, custom return and feed (Mugen makes a block and a reg that work together)
Custom header (DC Sports...etc)
Bigger fuel pump(?)
It gets pretty involved, and I believe you can get and interface harness from Hasport that clips into your core harness. A JDM ECU or KPro needs to be used because most USDM K20 computers have an imobilizer built into the key. Unless you have the key from the motor you'll need to have the ECU reflashed to get rid of the imobilizer.