Since the statute of limitations is determined state by state and since if this happened, it was outside the U.S. what are the statute of limitations? In many states it is 10 years if the event was reported within a certain time. My guess is this situation is beyond the statute of limitations. If Al Franken cannot be tried in criminal court and if he does not admit to it, he cannot be proven guilty.
The question is, should he be thrown out of Congress on an unprovable accusation? Maybe some people think that would be an act of social justice. I'm not so sure about this. Social justice could be a dangerous place to go, when it relies on opinion and not on the facts of a situation.