After several issues w/head gasket on my 99 SF 5 model S, I replaced the engine w/2.5l Impreza 2011 engine. I had to reuse the original intake/throttle body. Now I'm getting a misfire in #4 cylinder; I replaced all the wires/plugs and ignition coil modular and still have the misfire. Any recommendation as to what to check; an internet/ forum search has suggested the O2 sensor?