It can be done easy.
I read about some whitewater canoers who were on a multi day trip down a river, on one of those days they hit a rock and severely deformed the hull, think it was rolex so plastic, they built a large fire and held the dented section close the fire to get it hot, and then the dent just disappeared because the plastic had memory.
It’s been a while but here’s an update. I got the kayak uncovered now that warm weather has arrived, and tried doing some straightening. I used the car coil spring air bag described above pushing outward from inside, plus a couple of wood panels and a wedge. I used a heat gun to apply some local heating to the outside in areas that I wanted to deform. That worked well. I kept the heat gun moving constantly and heated until I could just detect a “hot plastic” smell. But there wasn’t any sign of the plastic wanting to change shape due to memory. It only moved if I pushed it.
Here’s a picture of the setup showing the air bag and wood with a wedge pounded in at the bottom after the air bag was pressurized. I didn’t measure pressure, but based on the resistance I felt from my hand pump I’d say 30-35 PSI. The black tube is the air line. The light tube is the skeg control.
Here’s the improvement so far. I’m letting it sit for a couple of days with the hardware in place but depressurized to see if it holds the new shape.
The coil spring air bag works well for this because it can apply a lot of force if needed, but the drawback is that the air bag is quite rigid even without any pressure. I used a large wood clamp to squeeze it just to get some air out in order to push it a little deeper into the stern. The air bag is also expensive if you don’t find one on closeout like I did.