It's a really good question...
Would you want to buy the canoe and then remove the glass to re-canvas? If the answer is yes and unless they are practically giving it away it's simply not worth the effort to remove glass unless the canoe is a rare and desirable one...in my opinion.
I've passed on quite a few canoes that would have been very desirable if they had not been fiber-glassed. It's just not worth the effort. Is it doable? Yes, generally it is, unless they used West Systems epoxies or something equally impossible to peel off. Glass jobs done in the 60's and 70's can be fairly easily removed with heat. If you are lucky and the resin has not wicked behind the ribs and between the planking it is possible to remove it in a few tortured hours...or you could spend a hundred hours with a dental pick, pocket knife, ice pick what have you to get it cleaned up enough to canvas. Been there done that and have one in the garage right now that I am trying to strip.... unless it's a very rare boat its not worth the effort.
If you want an Old Town, there are quite a few available, many of them restored or restorable. You would be better off waiting for one of those if you want to avoid the extra hassle.
One thing to keep in mind...when boats were glassed, often it was because they were too far gone to bother with restoring. A glass job can hide split and rotted out stems, bad planking etc.