Hi Scott
Now the VERY last thing I want to do is to pretend that I am an expert, but expertise more often than not comes from experience....!
So with that in mind here is my sixpence worth based on 'my' experiences.

The crank cases on the MC21's motors (I have 2) are pretty frightening to look at first for time. They appear to have loads of little 'hair-line' cracks all over in the crank area. Yours doesn't look any different to mine.
It has to be a pretty big BANG to do any serous case damage...i.e..rod letting go and seeking the outside world. In a battle between alloy and steel, steel is usually the winner!
Carefully clean the 'outside' of the crankcase and check for any damage there. I feel sure that you will be okay but check VERY carefully using an eyeglass if possible.

The crank, in my paranoid opinion, is a different kettle of fish though. Have it rebuilt with new bearings and seals. Don't take a chance if you intend to keep the motor.

As I said these are just my opinions based on my own experience which is based on several years of two stroke engine building..... BUT not much with NSR's. I'm learning all the time though.

Mike
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I don't mind dying...I just don't want to be there when it happens!