It would certainly be preferable to get them, but if they're not available I think Telltale will be able to cast decent sound-alikes.
Fox's voice has lost a quite bit of expression over the years due to his Parkinson's.
The guy who did the cartoon voice
did a good job, and Dom Armarto's name has been bandied about on the Telltale forums as a possible Marty if Fox is unavailable.
In the cartoon Dan Castellaneta voiced Doc. Heh.