As has been stated all it really needs is a new muff made with a wider gear on it. Get God knows how many of them made and supply them to anyone with a Trainorama loco that clicks etc. If it comes with instructions on what to do then most of us would be able to do it and those that can't do it should be able to find someone to swap them over. Be a lot cheaper than all new bogies though. Be a lot easier as well to fix it, rather than having to remove a whole bogie and fit a replacement or a new mech under it.

One other thing I would have thought about the muff on the GM's etc I would have thought that there would be a solid  plastic divider in the centre of the muff to not only stop the ends of the axle touching one another and shorting out, but also to provide a bit of strength for the gear itself as they would take a fair strain at times.