Can you fix a crushed (by a tree falling over it) fiberglass canoe?

It’s a lost cause. It can be fixed but you’re much better off getting a new one.

^^ Agree

How bad is it? Cracked is one thing, holed is another.

Cracked—a tree fell on it, so I haven’t yet seen, but scrunched and cracked in part.

What I mean is, the tree is still on it, so I haven’t seen how bad it is, but I saw a crack in the boat.

Yes. I have repaired broken fiberglass fenders on trucks.
Get some fiberglass resin, and some glass cloth, Grind out the crack some, using breathing protection, and a grinder, apply resin and one or two layers of cloth, apply more resin, let dry. Grind or sand smooth, fill low spots with more cloth/resin or body putty, Sand smooth, prime and paint.