I would bet that it is the mainshaft that has snapped, not much you can do about that really but it is a cheap fix if it is that, just rob a mainshaft off any cossie compatable MT75 and its about an hour to swap them...

The big problem with any MT75 derived upgrade is that you are limited on the physical size of components by the original casings, hence why the mainshafts are the weak point now on the Bara/fixit/dome gearboxes.