A General Motors Corp. subsidiary will buy an insurance holding company subsidiary of Xerox Financial Services for an undisclosed sum, the companies said Friday. Motors Insurance Corp., a subsidiary of GM's General Motors Acceptance Corp. unit, will acquire NAVCO Corp., a subsidiary of Xerox's Crum and Forster Inc. organization, the companies said. The deal faces regulatory approval and a vote from each company's board. NAVCO provides automobile insurance mainly to members of associations. It had annual premiums of about $147 million last year. Motors Insurance had revenues of $1.1 billion in 1989 and insures personal and commercial coverages and plans. Crum and Forster, based in Basking Ridge, N.J., also said it wants to sell its Viking Insurance Co. of Wisconsin. That company provides personal automobile insurance.