What ^^Chuck said. In the case of these Kughn-era Dash-8s, the motor field is staked to the truck, so you can't easily remove it; thankfully you don't need to. You can take out the armatures and brushes. I think the powered models had "LionTech" electronic E-units. I'm not sure whether these E-units will freak out if they don't detect resistance from the motors. To prevent any damage, I would probably disconnect the center rail feed from the pickup rollers and tape it off. The gears in the truck will turn with a little bit of resistance, but for all intents and purposes it will now be a dummy unit.
Be sure to save all the parts, it's very easy to reverse these modifications if you ever decide to sell it.