First off, Obi-Wan would have sold Luke's speeder AND C-3PO just to afford a pilot that would fly him and R2 to Dagobah, a planet that isn't even on any of the charts. Secondly, by the time Obi-Wan and Yada made their plans and head off, Alderan would have been destroyed, the Princess would have already been executed, and the Death Star would have begun destroying all systems opposing the Empire. Even if the old Jedi could locate the Rebellion, only a Force user could have made the shot that destroyed the Death Star. If Obi-Wan or Yoda was piloting the lead assault craft, Vader would have sensed it and obliterated them before they could even reach the trench.

The Empire wins. The end.