The 620 ECU! You will need to modify the loom of the 620 to fit the ZS or modify the loom of the 420 to have all the connection sof the 620.I would guess you use the 620 ecu as the 420 wont know how to fuel for posative pressure!
What other things you need depend on what you have now, is it mounted on engine mounts (all of them) with a functioning cooling system?
It wont run properly on the 420 ecu, it will fuel and spark incorrectly and will register boost as manifold vacuum colapse, it sounds like you are a little out of your depth.so far everything off the 420 loom has plugged into the engine. the only thing that isnt there is the smaller of the 2 plugs for the ecu.