Most of us here prefer a canvas cover to fiberglass, and if the canoe were in need of restoration, most would recommend replacing the fiberglass with canvas.
But if the canoe is generally good shape -- good, sound wood, smooth glass -- few would recommend stripping the fiberglass off just to return the canoe to its original state of being covered by canvas.
As to impact on value --
How to value a canoe has many variables -- Without knowing the canoe’s overall condition and its location, for example, it is difficult to make any kind of comment. For a discussion of determining value, see the post at:
http://forums.wcha.org/showthread.php?57
See also:
http://wcha.org/legacypages/catalogs/old-town/guide/
Canoes end up glassed because some people prefer it -- others won't touch such a canoe with a 10 foot pole, so among the other variables in valuing a canoe is the attitude of a potential buyer -- virtually impossible to determine in advance. But the canoe's present location oin Central PA would make it easier to sell than if it remained in distant Patten, Maine.
You may find these discussions of interest:
http://forums.wcha.org/showthread.php?9902-Fiberglass-question
http://forums.wcha.org/showthread.php?12167-Old-Town-Guide-16-restoration&highlight=keel
And as to measuring a canoe's length -- it is the distance between the extreme ends of the canoe -- usually points along the outside curve of the stems, where the stems curve back towards the interior of the canoe (as on Old Town's HW model), but sometimes the very tip of the stems, where there is no curve back. With the canoe on the ground, measure between two plumb, upright sticks placed on the ground outside the canoe touching the extreme end points of the canoe.
Greg