A hairline crack.... as in, the carburetor hit something and broke out the engine? Or as in, there's a gap at the O-ring mounting?

If there's been a mechanical leverage on that carb, to open a crack in the block, I think you'll have some deformation inside. Some engines need a tight clearance for the crankshaft to spin in, under the carb.

If it's just a casting flaw or an undersize O-ring, all you need is some Hi-Temp Silicone Sealer. If it's a deformed crankcase, you might want to run it on a test stand after you fix it, for a few tanks, to make sure there's no interference. Or, to make the flaw exhibit itself.

Good luck,
Dave Olson