I dragged my tube frame tiller over beside my large frame, and you are right, the tines, hubs and gear case appear to be nearly identical, with only the mounting arms being different.I have a tube frame tiller that I use for parts for my large frame tillers (I have two, one of which is in excellent shape). For the most part, the parts seem interchangeable. Also, I have looked at the mounts and it seems like the gearbox is the same, like you could just change the mounts and it would work. Don't know for sure though.
Perhaps the flange from a smaller tiller will fit billhuser's tiller?
Although, the parts manuals list very different part numbers, so I suspect a shaft size or spline difference will preclude interchangability.