That is a not so easy to answer question. If it is before the electors meet, they could vote for just about anyone, or could stick with the names on the ballots. If after the electors meet, then Congress simply votes up or down on whether they are legal.
If Congress boots him, fat chance, House picks the president (one vote per state) and Senate the VP.
Remember when Biden kept saying how he’d be proud to serve with McCain......Even SNL got into the act.....Well?