Thias
Klein Fan
That kind of dent in a steel frame is no problem stability wise, imho.
I'd try and massage it out using a hardwood block with a hole in it the exact size of the tube, then sawn in half. The dent has made elevated "waves" on its side. If you slowly work them back down, the dent should "move" back out, too. It won't be perfect, but still. Oh, clamp the wood block in a vise, with the dent covered by the wood. Use little clamping force at first, turn the tube to "roll" dent smaller. Then tighten and repeat.
I'd try and massage it out using a hardwood block with a hole in it the exact size of the tube, then sawn in half. The dent has made elevated "waves" on its side. If you slowly work them back down, the dent should "move" back out, too. It won't be perfect, but still. Oh, clamp the wood block in a vise, with the dent covered by the wood. Use little clamping force at first, turn the tube to "roll" dent smaller. Then tighten and repeat.