1983 engine going into 1989 mustang lx

You will have to remove the distributor, lower intake with injectors, the harness (basically everything bolted to the 89 engine minus the cylinder head), swap it to the "new" engine. IT might look identical but it's highly likely that it isn't. Better to swap it while it's on the floor or stand than the fight it in the car. Is the 83 engine a turbo engine (does it have a fitting in the passenger side of the block?)

By the way, a 8993 head gasket (for 2.3) is like $6.