Financier Carl C. Icahn reported to the Securities and Exchange Commission that he has increased his stake in Trans World Airlines to 47% from 45.5% by buying 742,000 shares at $22.50 per share . . . Hilton Hotels and Unocal have agreed to accept Sears' new Discover Card . . . American Motors lowered its discount car financing offer to 7.5% from 8.5% . . . Ryder System and Aviall Inc. have entered into an agreement providing for Ryder to acquire Aviall for about $125 million . . . United Technologies' Mostek Corp. unit said it will lay off 500 workers, effective today.