Great looking machine! I think it's time to take it out of the shed, clean it up, and put it back to work. I love Hamilton Beach motors like yours. That conversion is early enough I would leave it just like it is.
If you do want a hand crank I think I see a motor or handcrank boss on the back. If that's the case hooking it to a handcrank would be incredibly simple. The only difficulty with treadling it is most treadles were made for full size machines, yours is 3/4 size. If you can't find the correct treadle base, you would need to adapt one to fit.
Rodney