Two Stroke Pipe Dents can be removed.  The "freeze method" is where you fill the pipe partially with water, when the water freezes, it expands, and can have an effect on removing the dent; however, you must be very careful.  You could destroy your whole pipe using this method.  We recemtly purchased a product that allows you to plug both ends of the pipe, and fill the pipe with 60-90 psi of air pressure.  You then take a propane torch and heat the dented area, and the dent slowly starts to come out.  You may have to polish the pipe afterwards; however, we have found this method works very well.  Use caution, as air and flame may not be a good combination, but hopefully safe enough to take out most of the dent.  We offer two stroke pipe dent removal as a service.   I would be happy to give you an estimate and fix your pipe for you.  We would need to work out the shipping details and find out if your pipe has any holes in it.  If the pipe requires any welding to repair holes, so that the air pressure may be properly applied the cost is slightly more, but reasonable.  Please e-mail me anytime with any questions you may have.