liilpa09 ,why not disassemble the engine and bring the left case to the best welder in town? I am guessing the price of gaskets and welding will be a fraction of the price of a used motor. Or if the crack is very small, you could try retapping the hole to a larger thread size and use a larger aluminum sealing washer, with the hope of the larger washer will cover over the crack. And on a last resort the crack could be grooved with a die grinder and jb weld epoxy applied to the crack. You would be amazed how well JB weld sticks to a rough clean surface.
Any way you choose, I hope you get it up and running soon. Let us know how it works out. Oh and by the way, I have found my local Acura dealer has a great selection of aluminum sealing washers from small to large, I use them all over my Isetta. John