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You will probably need to give it a full service, check the drivetrain, clean out any old grease in the gear towers add new lubricant to gears and wheel bearings, clean all the wheels check the wiring and pickups too.
Does it have flywheels on the motor? How easy is it to spin them by hand, or it isn't and that what you mean when you say "feels stiff"? That will indicate the degree of the problem due to drive train. It should be easy to turn.
If it is easy to turn, then what does the motor look like? It is one of the ones that has open sides, or is it enclosed in a rectangular can? The open frame ones could get a coil wire short circuited or burnt through such that the electricity isn't traveling through the whole thing. Heat and loss of power in that case.