The two big screws remove the roller plate and allow access to the wheels and armature. I would flush the whole thing with CRC cleaner a few times over paper towels or shop cloths. Let it drain and dry. You want to use light weight oil very sparingly on the wheel bearings, side rod connections and a drop on the armature axel. The gears should get a little white lithium grease. Use a toothpick to apply. Check the wire insulation. If it is rotted, it has to be replaced. There are sources online for cloth wrapped wire. Careful with the wheels. They are quartered, which means they are set at the correct spacing to allow the side rods to move freely.
This was my first standard gauge engine. I messed it up fixing it, but I finally learned and got it running. I love it because of all that. The e-unit can be finicky. I locked mine in forward with a zip tie around the lockout lever.
They are pretty durable, so you shouldn’t worry too much about hurting it. After putting it back together, you should be able to spin the drive wheels to check that nothing is binding. Post back here if you have any issues.
George