Guys,
I might be very lucky in this, but the same shop that will grind my crank journals will also spray weld and fix splines- you might be best off talking to an automotive shop, or might even be best to talk to a shop that specialises in large diesel motors, like earth movers or semi-trucks. I know those earth moving equipment crankshafts cost an absolute fortune, and fixing any problem is much more common then replacement.
They may even enjoy working on a crank that doesn't require a very big hoist to move around, and give a good hearted BSA owner a price break!
Lee