On the eve of his scheduled sentencing in Los Angeles federal court for counterfeiting, Jack M. Catain Jr., a 56-year-old Tarzana businessman and reputed organized crime figure, was hospitalized with a heart problem, his lawyer said. The lawyer, James A. Twitty, won a three-week postponement in Catain's sentencing from U.S. District Judge William P. Gray, who remarked: "I wouldn't put it past Mr. Catain to pull a con job. . . . However, I'm also convinced--and it may be an understatement--that he's not a well man."