I heat the area that I want to bend with the heat gun, it will get "soft" then I bend it so it is straight. Heat it till it is about too hot to hold. In the pix of the heat gun you can see a piece of 2x4 that has one side cut in an arc. I just bend the bent area over that arc to straighten it out. It takes a little practice but once you get it, it works nicely. One other thing to watch out for is crepe myrtle has a "pith" in it so you will need to remove wood evenly around the shaft so you keep the pith in the middle.
Jack