You could try looking for a RV recycler (junk yard), as this type of jack is also used on small pop-up campers. Personally, I would go to Harbor Freight or look through my extra tools and find a socket that fits over the shaft that your broken crank is attached to. You remove the handle, drill a hole in the socket that matches the size of the hole in the crank shaft, and then bolt it to the shaft. You can now either use your ratchet or a portable electric drill to activate the jack. Best of luck.
Probably be easier to just replace the jack.
Here is side frame mount tongue jack
I'd personally go to if not electric, then an A-frame style tongue jack
The Bulldog A-frame one is really nice
Our Trailers:
2015 19 Escape
Buying or Selling Molded FG Trailers:
Fiberglass-RV-4Sale
You could try looking for a RV recycler (junk yard), as this type of jack is also used on small pop-up campers. Personally, I would go to Harbor Freight or look through my extra tools and find a socket that fits over the shaft that your broken crank is attached to. You remove the handle, drill a hole in the socket that matches the size of the hole in the crank shaft, and then bolt it to the shaft. You can now either use your ratchet or a portable electric drill to activate the jack. Best of luck.
Dave & Paula
https://amzn.to/2LZxGeX
Eddie