Financier Carl C. Icahn sweetened the pot for shareholders of Trans World Airlines in his quest to take the carrier private. Icahn, who is TWA's chairman, already owns 73% of the airline's stock and wants to buy the rest. Under the revised offer, stockholders would receive $20 in cash and $25 in face amount of 12% subordinated debentures due in 2007 for every share held. When Icahn originally made the offer in July, it was $20 in cash and $20 in debentures. According to TWA's general counsel, Mark Buckstein, the offer was raised to obtain the support of investment bankers. The TWA board approved the offer Friday and it will be presented to stockholders for a vote.