Modified suspension, bodywork and brakes are also installed, but more of that elsewhere. A Chevy, with adaptor, about to be dropped into a Nissan MK Patrol. Manufacturers go to a great deal of trouble to find the best compromise in terms of performance, tractability and reliability for their vehicles. 4WD engines have to be flexible and capable of running at extreme angles. Car-type engines may be great on the highway but not so good off it.
Some insurance companies are also suspicious of anything non-standard.
The chance is often taken to install a more powerful motor that is locally available. That may require other components to be uprated and suddenly it is not so cheap any more, but it will put new zing into the old truck. Engine mounts will match the motor to the chassis. A longer motor may necessitate moving the radiator forwards which may also require surgery to the chassis or front bodywork. 4WD radiators are usually heavy duty and may serve a larger engine, but a clean-out is a good idea.