One possibility that you could consider would be to find out from the guys here on the forum what a similar boat in wood/canvas construction should weigh (an 18' Otca is usually listed at 80 lbs. but I don't know how much more the sponsons add). A catalog might be another source for that information, though catalog weights sometimes seem to be somewhat inaccurate and seldom err on the heavy side.
Then weigh the canoe in question. A high-quality fiberglassing job shouldn't weigh much more than the canvas would at all, and a really well done fiberglass covering will often weigh slightly less. High-strength fiberglass is a fairly delicate balance of fiber content vs. resin content and unlike building a stripper, covering a rib and plank hull also involves plugging a lot of potential leaks where there are small gaps between the planks, or figuring a way to bridge them when applying the fiberglass skin. This is by no means easy and makes glassing a rib and plank boat and doing it well, much more difficult than glassing a stripper.
There are certainly some composite-covered R&P hulls out there with well done fiberglass skins, but high-quality, non-professional (or non-factory) glassing jobs are pretty rare. The cure-all approach that is frequently seen to address this tends to be slathering on a whole bunch of fiberglass and ending up with a seriously overweight canoe. Most of us have seen plenty of these over the years. If you weigh the Otca and it turns out substantially heavier than what wood/canvas should weigh. It well may be one of them.
In that case, the best course of action is probably glass removal and re-canvasing. Removing fiberglass is a tedious, nasty job that can't be rushed or it rips the wood apart. Unless the canoe is either in really good shape (wood-wise) or something rare and valuable (which a 65 Otca is not particularly, but I'm not sure about a '65 with sponsons) then $1,500 to buy yourself all that extra work doesn't strike me as much of a bargain. In a case like that, I'd save my money and look for something that's still wearing its original skin.
