I have used the heat gun, and open flame on past quads and has gotten rid of the stress marks. You just want to make sure you have the piece supported so when it heats up it wont sag. I've seen lots of people who didnt support there fenders, heated them and they sagged, and then they tried to bend back into shape only to look like ****. I had to use quite a bit of heat to remove marks. Basically what your doing is melting the plastic a little. Just becarefull that you dont blister the plastic by holding the heat in one spot to long, just move it back and forth across the spot.
