would a 2002 Ford Explorer SUV 4.0 engine fit a 2003 Ford Sport Trac

Yes, it will fit but check the emissions laws in your State. Some do not allow a donor engine older than the model year of recipient car, must be same year or newer and have all emissions parts come with

Make sure the engines both fall under the same smog requirements. A one year difference might not matter but check first. I would get the computer and the wiring harness that go with the replacement engine.

I'm trying to see if anyone knows of an engine that would be compatible with a 2002 Mercury Mountaineer or interchangeable thanks in advance

I have 2005 mercury mountaneer 4.0 I would like to know if the motor from 2002 4.0 fits on it with all,the harnesses?

Besides emissions laws! The difference i would see would be the intake manifold seeing it being a different design. Also the 02 has a mechanical throttle body and 05 newer has electronic throttle body. 60 degree engines are a pain to work on if they need timing chains because you have to remove the trans to change the rear chain, so replace or check before install.