Get the case half repaired, good or new Left side crankcases are hard to find, rare and expensive....Posted in a different post:
As Martin stated; easy fix...!
Tear engine apart, clean well, de-grease, have it TIG welded, then machined-blended. Then lap the case half flat. It will turn out perfect if done correctly. Do not TIG weld the broken piece back, the gap-area needs to be filled completely. If you weld the broken piece back, you will get voids, porosity, and very poor adhesion & strength.