mercury 400 questions

Hi everybody, I'm new to this boating stuff. I've never had a boat before and I recently acquired an old aluminum boat with a 400 mercury. This boat was left in a field for 4 years with the engine off and lying in the weeds. It cleaned up good though and is not seized. The previous owner who took it off can't remember why or how anything hooks up. So I have a couple of questions. First of all it has remote controls. Two levers. which cable goes where? They both seem to work in unison. when I push the throttle (bigger lever) to a certain point it also moves the other cable. Where it connects to the engine, one cable has a black end and one has a silver end, if that helps. I've tried hooking them up both ways and either way they jam and I can't move it at all.
Next question. The wiring harness has been cut at the end where the key switch would be. I'm sure I could wire some switches to it with a push button to engage the starter. My question is which wires need to be connected together to complete the circuit, and which wires would I connect to the push button switch? They are black red, blue, green, white and yellow. I found a schematic but that doesn't help me determine how to wire a toggle switch or switches.
I took the carbs off and cleaned them out. In my experience with 2 strokes (snowmobiles) the air intake is very important in relation to rich or lean running. there was no airbox on the carbs. Do these engines need airboxes with specific cubic inches or is an air filter all I need?
Thanks in advance for any help. I'd really like to get this in the water soon.