What makes you think the crank journals are worn? Are they scored, burn't or galled. If not your way over thinking this, get new bearings or used ones and plasti gauge them. I bet you'll find it's just worn bearings. I've rebuilt well over two dozen KZ650 / 750 engines and never found a worn out crankshaft even on abused ones. Most didn't even need new bearings. I simply plasti gauge the old bearings and if they ars in spec I reuse them.
Actually the KZ650 had a soft journal surface. When these motors were subjected to big power boost, the journals would die. Back in the day, when people raced these bikes we would offer a service where we hard chromed the journals not make them live. It is the only crank we have ever had this service for.