2003 Jeep Cherokee

Engine Mechanical problem
2003 Jeep Cherokee 6 cyl Four Wheel Drive Automatic

We recently fitted a 2nd hand engine to a 2003 Jeep Cherokee 3.7 sport. The previous engine had overheated due to a radiator leak. Now the "new" engine had a different torque converter and intake manifold which we changed. The ring gear to which the torque converter is fastened was different as well and we changed that. The thing is now the engine doesn't want to start. It cranks and it seems to want to start but then just backfires through the intake and dies. Compression test was done as fuel pressure checked and spark as well. Checked D.T.C.S and there are no fault codes present. What else can we check?