can a ship that is ioned before it's combat phase, still attack?

YES!

It still gets to do everything it normally could do. Unless some effect is specifically looking for Ion tokens the ONLY thing being Ionized will affect is the next time you would reveal your maneuver dial. If you would reveal your dial, or do anything triggered by that such as use Advanced Sensors, you skip that reveal and instead do a white one straight after which you remove ALL Ion tokens and go on like nothing has ever happened.

The above happens when a small ship has one ion token, or a large ship has two ion tokens. If a large ship has only one ion token, it holds onto it until it gets a second one.