Bad luck! That hole look really bad.
Do it properly and avoid further problems. Pull the engine and find another one as replacement. If you have invested in fresh cylinder, ported /restored head, cam etc in the old you can use them in the replacement engine. Replace the chains and tensioners, seals etc when you have it apart.
If your old is not restored, just replace it with a good old one and no need of restore/upgrade right now.
You can probably find an entire good engine for the cost (or less) to weld the old cases and machine job
I was lucky back in 1988 when I broke my chain on Autobahn in Germany during a holiday trip going south. I geared down to 3:rd gear and gave full throttle in an uphill to pass som slow going truck when chain just snapped. Chain had too much slack. Strange, no hole in cases, I was sure that should happen. Chain cut a piece off the registration plate, chain case brackets on swing arm broke when chain guard got hit.
I had moore luck when 2 guys on a small bike stopped and could bring me to the nearest town to buy a new chain just before closing time in the afternoon. Brought me back and bike worked again. Chain guard dumped in a dumpster later next day in France
I had to find anoother swingarm 2013 with brackets for chain guard, plus the chain guard as well that I had to order from Japan, yamiya750.
Maybe that the smaller front sprocket saved the cases that time, 17 teeth. Chain was a DID 50HDS O-ring.
I put a really strong chain last year when resoring the bike, still 17-48t
I got an 18t sprocket today when I'm tired to hear the squeeling engine. 180km/h feels like 200+.
