Check the foot controller which go out of tune after 60 years. If you have a good spare, try it out. If not, you could make sure the copper bridge is making contact with both contacts (it's obvious what I mean if you look inside the FC, or Google it).
If the FC is okay it's easy to rule the motor out next. Just undo the stop motion screw and see if the motor will run at full speed.
If you rule the motor out, turn it over by hand and see if it feels stiff.
If it does, you can find the problem quickly enough by turning it slowly while watching each part move.
If it's consistently slow and not the foot controller, it's likely just in need of oil (somewhere it wasn't soaked).