By Jan Strupczewski BRUSSELS (Reuters) - Euro zone finance officials were struggling on Tuesday to find a way of keeping Greece from defaulting on debt repayments to the ECB next week, with up to six options on the table but none that is problem-free. Greece needs 3.5 billion euros ($3.9 billion) by Monday to redeem its maturing credit from the European Central Bank. Before this is paid, however, Athens has to settle an overdue payment to the International Monetary Fund of more than 2 billion euros.