Somewhere hidden in the archives here is a thorough discussion of balance factors. If I can find the thread I will post it here, but a lot of the discussion was around moving to lighter pistons. Essentially 850 pistons on a 750 crank would be moving the other direction.
As for the keyway. How much damage is there? I have done a redneck repair (not on my Norton crank, it was a Perkins for a timing gear) with my Dremel tool and widened the slot from 1/8 to 3/16 in both the crank and gear and drove away afterward. If you use the undamaged side as an index you can get it pretty spot on. The 180-degree out repair as mentioned above is a good solution, but widening the slot is cheaper and can be done without tearing down the bike. FYI, I used the fiber reinforced cutting wheel and not a grinding disc to cut the crank, and a file and lots of patience on the gear. Admittedly, this repair was done out of desperation to get a boat home from a remote location and was not done without trepidation. But it turned out clean and it worked well. So there ya go.
Of course if you tear the bike down, you could use the 750 cheek on the 850 flywheel.
Russ
