indentations can be taken out of a tube from the outside with a block having the outside diameter of the tube being the bore of the inside of the block.. the block cut in half and clamped around the tube.. greese the tube and the block and as you increse the preasure on the block with a clamp, rotate the block around the tube..
At one time, I was was a partner in an upper-end bicycle shop (Corsa-Cycles) and every now and then we'd repair a bicycle frame where someone dropped it against something or it was involved in an accident
durring a race
and put a dent in the frame.. we would take the dent out with a wodden block..
As I said, when clamping the block around the tube, roll the block around the tube as you put preasure on tightining the block..
The idea is that if the dent pushes in in one part, it pushes out on another.. if you slowly roll the outer area in, you're also rolling the interpart out..
You might even go to a bike shop.. they might have the blocks in the back to fit the diameter of the tube you have..