My 2¢,
If it's hurt that bad, more damage is waiting if you split the case.
Better off getting another engine, trust me. You'll hopefully save money, aggravation, and time.
Rebuilding these engines is different from a car engine, and not worth it.
My PreGen engine went bad, a lot of bottom end noise, upon draining the oil, it looked like 70s metal flake paint, I didn't even bother going any further into it.
I just got another engine, granted it a complete Ninja 300 with wiring and ECMs etc...
As seen here
https://www.ninjette.org/forums/showthread.php?t=361054