Japanese trading house Itochu Corp. said Wednesday it has formally decided to help rebuild Bigmotor Co. after conducting due diligence on the assets of the used car dealership mired in an insurance fraud scandal. Itochu will set up a new company with its subsidiary and investment fund, J-Will Partners, in April and acquire Bigmotor’s used car and car-repair businesses, the trading house said, without disclosing the size of the planned investment. The founding family of the dealership, widely seen as having created the corporate culture that led employees to commit fraud to improve performance,…