Is the Speedometer connected to the tail shaft? If so then that would make a ton of sense.
The part that is busted is where the drive shaft goes in [bottom left].
You can see here that it is detachable [Bottom Right]
I wonder if a junk yard would let me get one of these off an old transmission, because I can not find where to buy it anywhere. That still leaves the problem of finding a gasket though. I could get an entire rebuild kit, but it seems excessive for just one gasket.