I have columbia 2-4-2 union pacific steam engine, model 91101. It jumps while going around the track. After farther investigation I found the drive gear is missing a tooth. Does anyone know where I can get a replacement?

I'm new to these trains, I inherited a royal blue 4-6-0 train from an old friend. If I put power to the pilot trucks the wheels turn. If I put power to the rear wheels  the wheels turn , but when I put it on the track nothing happens unless I lift the front or rear wheels off the track and then they turn. It looks like the pilot truck was reinforced, maybe rewired. haven't taken apart. anyone have any ideas?