The first thing that needs to be done is to clean the track, locomotive wheels and pickup rollers. Use a ScotchBrite pad to remove rust from the tracks and any that may be on the wheels or pickup rollers. After all the rust is removed, clean everything with Naphtha to get rid of all the dirt and grime.
The reverse unit is a 2-position unit - forward and reverse only. Very little goes wrong with them.
Put a drop of oil on either end of the motor armature shaft to lubricate it, a drop of oil on the wheel bearings and a dab of grease on the gears and you should have a nice running engine.
You can do this at home and save yourself a service charge and learn how to service your equipment. That is what I did.
The service manual pages for your locomotive are here
, pages 3-11 and 3-12.
Let us know how you make out.