Well, in 1997 and 2001, she endorsed Hague and Duncan Smith, which was certainly a factor in their election, but she didn't endorse a candidate in 2005. Say she did--who does she endorse? David Davis? Possibly, but that probably doesn't change the outcome, given the scale of Cameron's victory. More interesting might be if she endorses Liam Fox--apparently, he was the candidate the Cameron team were worried about getting into the final round, and he was only six votes behind Davis in the second round. A Maggie endorsement could be endorse to make the popular vote round Cameron vs Fox.
I'm really not sure whether Fox can beat Cameron, but I'd expect him to do better than Davis--Cameron and Fox both have a populist streak to them that Davis really doesn't. But I'd say he has a chance.