I've been playing with the Roomba, as it's having problems due to a little pin dropping out - it looks like it's now damaged the plate that the pin sits into. I tried putting some duct tape in to hold the pin in place (it's sliding through the back of the mounting), but it punched through that quickly.
While you can buy the whole gearbox unit, I thought I'd try epoxying the pin in, so it won't move any more, and the gear will spin on it just fine. I had to take far more apart than I have previously, and I've left it to dry overnight. Hopefully this fixes it once and for all.