1985 318i

I recently bought a 1985 318i BMW coupe (2 door). ($400.00 Cash). Ive been driving the car w/ no problems.. back and forth from Va. to NC . finally it needed an alternator. when i ordered the alternator (used junk yard), went to put it in myself.. (first time). found out that the alternator i needed was from a 1977-1979 BMW. I believe someone put the cheaper 1977 320 engine in my 1985, probably after that engine went out or something. I finally decided to get a re-manufactured alternator to avoid the hassle that a 30 yr old used part was probably sure to give me. I installed it and everything runs fine.
But my question is.. if there is a 1977 engine in my 1985 318i does that mean that everything, or just the motor is 1977. I mean like the Idle Control unit (in the glove box) would that be 1985 or 1977. or is just the engine the only thing i have to worry about .