Hard is a relative term. If you have the Pittman motor with the setscrew in the flywheel, it's simple. If you have the motor with the flywheel pressed on, the flywheel may have to be pulled and the retrofit flywheel and magnet kit installed. The trick is pulling the flywheel, you can't pound on it to remove it, you need to use a wheel/gear puller. I don't know for sure what that locomotive has internally.
Edit: I looked up the Lionmaster Challenger, it has two motors with the pressed on flywheel. If the magnet is cracked or broken, the flywheel has to come off. Below is a picture of the unavailable motor with the flywheel magnet, note the magnet is installed from behind the flywheel, there's a rim that retains it. I'd normally recommend buying the whole motor, but they seem to rarely have them anymore.
