Those older Athearn rubber band drive locos were built in the 50's and 60's. They were notoriosly poor runners. It's great if you are restoring it for sentimental reasons but it will probably not preform very well. Good luck.
One of the biggest reasons for the poor performance was the poor quality track as much was brass. Wheel cleaning maintenance is a must, as they are also brass.
I have a number of the old "belt drive" Athearns and you would be surprised how well they perform on nickle-silver track. Not Proto 2000s mind you, but they can hold their own. Mine are from the late '50's and early '60's. All of mine are the old F3's, F7's, a C-Liner, a couple of RS-2's, and a couple of old PA's.
Those old Pittman-style motors are still amp sucking fools too. I tried to convert one to DCC to see if it could be done, and it fried a 1.5 amp decoder in quick order.