I've repaired a busted case on my old Suzuki GS1000. It had engine guards but the mounts turned out to be weak. On a slow gravel road one time a farm dog launched at my boot and hung on. I dropped the bike on the left side and the engine guard bent back and pushed into the stator housing. It cracked it badly and leaked oil all the way home.
I didn't take the cover off for an inside fix as I would have had to push out the damaged part with the risk of it cracking away, so I degreased and sanded the clear coat off the sunken area and filled it with metal epoxy on the outside. The repair lasted for years until I sold the bike, but the dull grey patch was evident to anyone with the motivation to go looking.
If there's a hairline crack the neatest fix would be on the inside. I'd grind a slight gap to give more surface area. I'd also go looking to see what made it crack as it might do it again. First place I'd look is whether the gasket surface is dead flat all around.
I've used Blue Hylomar for gasket sealing but I'm not sure if it could be rubbed deep into a hairline crack and expected to hold the seal long term. They say it's gap sealing, but I've taken that to mean when sealing gasket surfaces that are not perfectly matched. However, it's a much easier first attempt than pulling the cover etc.
