Tax evasion costs the Greek government an estimated $20B a year. I would say that improving tax collection would significantly help Greece make payments on their debt. You have a big problem when there are millionaire doctors not paying a cent in taxes to the state.
Sure, they have money wasting state enterprises, they could try to privatize some of them - but it is something that should not be done in a hasty manner. Rushes to privatize have led to massive failures (see Russia, parts of Latin America).
So in order to get out of this mess they should 1. Restructure the debt to get a longer payment schedule 2. Improve their ability to collect taxes so that they can make some of those payments. 3. Work to privatize certain state owned businesses / infrastructure.
... or drop out of the EU, devalue currency, grow GDP