Back in the "Williams by Williams" days, someone in our club was able to get new gears for the axles (just the gears). I think he got them from the same old man in Maryland that was mentioned earlier in this post. Another club member used a punch and vise to remove the worn gear from the axle and replace it with a new gear. Then he gave the truck to me for final assembly. We did this to about 8 -10 trucks. Only about half of the fixed trucks actually worked mainly because at the time we did not realize that there were two different gear types. One was for 6-wheel trucks and the other was for 4-wheel trucks. Of course, we were using the same gear for both trucks.
Now in the Williams-by-Bachmann days, I don't think you can get just the gears separately. As a matter of fact, you may not be able to get just the trucks either. Last December, Bachmann had a 20% off parts sale so I went on their parts web site and ordered a bunch of stuff for the club. As far as trucks go, I could only order the whole assembly which included both the truck and motor.
Even if I could get my hands on some individual gears, I don't think I would have the time or patience to go that route again. Since I don't charge the club for my repair time, it is still cheaper for them to buy the whole assembly rather than a new engine. And the repair time is a lot shorter for me.