Find a talented welder and have them TIG it. Should be able to fix both parts, but it may be cheaper to replace the left case cover than fix it (ebay) unless time is a factor.
Look at eBay and get another sidecase cover -- $35-$100. It will cost more than that to weld it and then clean up the cosmetics and mating surface itself.
Case itself -- strip paint around the area, clean and remove all oil and TIG weld up as required. Then cleanup the mating surface. Depending upon the weld itself, you could be able to cleanup the external weld itself and then lightly dimple to emulate cast surface. Paint as desired.