How to repair hairline crack in crankcase?

Hi. Had the clutch cover off to replace a broken bolt, noticed a hairline crack in the wall of the case near where the bolt went. Before the bolt was removed, the bike leaked oil pretty bad, now it is much better, but still seems to have a very minor leak. Not sure if this is beacuse I made a gasket from childrens craft cardboard, or because of the crack.

Will fit a proper gasket next time I change the oil, but I suspect the crack is what is leaking, and the bolt was broken by the previous owner trying to cure the leak with excessive force. Any ideas how to fix it? A new case is cheap enough, but if I pull the engine that far apart, I might never get it back together. Was thinking about soldering/brazing it, or finding someone who has TIG gear.
 
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