I actually was able to fix one of the K4 models with shims inserted from the top. It was a bit of a PITA, but I managed to keep the gear in place and AFAIK it's still running. The next one I think I'll pull the drive block and flush it with carb cleaner to get all the grease out, then use retaining compound on the gear to see if that works.
I'm going to check my H10, it hasn't run much, but I'd like to head off that issue before it happens.
I did a pig & a poke at one of these afflicted compound gear boxes,…what a hunk of junk!…I developed a fix for these, as long as the gears haven’t gone into mangle mode, pretty invasive, and a real clock bandit,…the fix involves pulling drivers, pushing out the pin, adding better splines to the pin, and a brass sleeve to keep everything aligned,….not one of their crowning designs,…..I’ve had a couple requests to duplicate the service, but both had destroyed gears, so no fix if the gears are wasted,….as John has stated,…it’s chassis time,…