The big danger with recovering a missing Doctor Who story is that it might turn out to not be as good as the collected memories and legends claim it was. The obvious example is Tomb of the Cybermen which wasn't quite as scary as everyone remembered it. However the photos that have survived of Marco Polo suggest it was a great visual spectacle and an early triumph for the BBC costumes department.
I know it would be the story that would most excite the legion of fans and they deserve a big find. Maybe some rogue Gallifreyan out there has recorded the transmission as it moves out in to space and one day it will be brought back to us.