Engine Swap
 

I have a 2003 mazda 6 with the 2.3 engine and 4 speed transmission the motor has over 300000 miles on it and its leaking oil pretty bad .I have a 2008 2.3 with 5 speed transmission i want to replace it with i have the harness from the 08 and ecm tcm also .My question is am i gonna have problems with the imobilizer because i dont have the ignition from the 08 and are there any other problems i might have

Use the 2008 bare engine long block.

Use the 2003 transmission, wiring looms, intake manifold, exhaust manifold & all sensors & auxiliaries.

Depending on specifics, you may need to swap the rocker cover and possibly camshafts.