I think it would have to be McCain. McCain would have been the only valid candidate in the race. It would be like the democrats had no candidate at all. If Oboma is Kenyan, he couldn't have run in the first place.
If Zero is ineligible, his election is VOID. Along with Biden, since he was on the ticket. Biden ran for VP NOT Pres. There would have to be another election process to determine who serves.