Looks like it'll be fine to me. If you check the film, Fox actually has a larger noggin compared to his little body anyways...and with all the layers of clothes and that 'life preserver'...I think we'll be fine.
I think a lot of the perceived head size in real life and in the sculpt is due to the poofy hair-do. If you look at the actual facial features the Marty head is smaller. For example, look at the distance between chin and nose:
I'm not really worried anymore.