You mean the bore that holds the shaft for the idler gear, correct? If so, it came in two ways. One way had a thrust bearing the gear rode on and the other is like you have, the shaft itself spins whilst the gear is keyed.
In short there isn’t an “easy” fix. It depends on what tooling you have and what your capabilities are. I’d suggest either broaching it and making the new shaft for the gear to spin on while it’s affixed to the tumbler; or reboring the hole and making a new larger shaft. You could also sleeve the bore but I’d expect your shaft is worn as well.
I’ve made several replacement shafts, idler gears, and main pinions for members here. If you don’t have the tooling to do it you can send me your tumbler arm and I can do the repairs for you.