HONG KONG, June 5 (Reuters) - SpaceX's website and IPO marketing documents were not accessible on Friday in Hong Kong and mainland China, a Reuters review showed, a step that threatens to curb participation by investors there in a listing expected to be the world's largest. Elon Musk's SpaceX aims to raise $75 billion, the most globally in an IPO, in a deal that would value the company at $1.75 trillion, immediately vaulting it into the ranks of the 10 most valuable U.S.-listed firms.