If the Court doesn't find enough proof to declare Trump the winner, they would probably send the election back to the states for a re-election.
This will never happen as it would be itself an unconstitutional act.
This would be a case of first impression.
The constitutional assumption is the President is there legitimately and what he does AFTER in office is outlined. The Constitution doesn’t contemplate a fraudulent election process. The Court would need to adhere to sound constitutional intent.
It seems to me that if there was enough of the right kind of evidence the Court could rule the election and its results null and void and take it from there.
To me, this is like a RICO (Racketeer Influenced and Corrupt Organizations Act) case.