No power it just stutters and misses hard

Ok so were driving and I just purchased jeep and I knew it needed exhaust manifold replaced however I didn't know it was completely apart from muffler. Anyway ordered manifold and for a month now I've been driving and other than it being extremely loud it runs really good. I keep the oil clean however it has run out of gas a few times due to the gauge not working properly anyway now it all the sudden stutters and misses big time as I'm driving and really has a huge loss of power I should mention I've put fuel injector cleaner in a few times as well as Lucas motor oil treatment and lastly the air filter is dirty and wasn't even in the previous owner totally neglected this jeep and was extremely lazy when it comes to anything to do with maintenance on jeep. He claims he forgot to put air filter back please help me as I have children and its all we got