For the one pictured here by Oldfart. An easy foreign order, repair for a fitter/machinist/plant mechanic, in a general workshop.
Thread is good, although an easy repair, if not.
Weight wise, good second hand gears are cheap to post to a crank in comparison and come up regularly on US ebay. See if still available new?
Just weld the crank shaft damage up. Rip back to within a ten of a thou, to produce a nice K6 transition fit for your new second hand gear. That key-way was and is an easy cut in a milling M/C(not open slotted). The trick is to cut it into the opposite side of the shaft to the damage, into the original undamaged parent metal.
Key fitting is another trained skill, not learn't from Wikipedia. The key fit must be precise. 243 Threadlocker on the nut will be fine and allow easy future servicing.
If you are not confident with your key fit. Use 601 bearing lock on the key-way and shaft.
Thoroughly clean and "prime" all surfaces if you want the loctite to work correctly.
from memory, I believe the crank seal runs on the back on the gear, and is therefore the renewable surface. There will be no need to harden or grind anything.
GUARANTEED.
Of course a good second hand one at the right price is also a real good option.
