MX250, I restored a Kawasaki KH250 (the little brother of the family of two stroke road bike triples built around the early to mid seventies) about 10 years ago and it too had damaged exhaust pipes. Apart from the scratches, all three pipes had dents of varying depths.
I managed to repair them by sealing the pipes at both ends so they were air tight then using regulated compressed air to gently pressurise them, and the apt word here is gently, while heating the affected area with an oxy torch.
To do this I made a couple of ply wood discs that were bigger in diameter than the inlet and outlet hole at both ends of the muffler (without the header pipe) then glued some rubber matting approx 3-4mm thickness to one side of each so they could be used as air tight seals. The two plates then have to be clamped so they hold tight against each end of the muffler, (so the air doesn't escape).
To do this is a matter of your imagination and the design and specs of the individual pipe you're working with, but I drilled a hole in the middle of each plate and fitted an eye bolt to one with the eye facing inward (the rubber side) and attached an air fitting to the other plate. To clamp them to the muffler I ran a length of wire down through the muffler and fitted /clamped one end of the wire to the eye bolt and the other end to the air fitting (you may have to drill a hole through the fitting to attach the wire) then tightened the whole assembly up with the nut on the eye bolt so it became a sealed unit (place some rubber under the nut and it's flat washer so you don't loose your seal).
Next step was to attach the regulated air supply and slowly increase the air pressure and heat the damaged area with the oxy. Too much heat and air pressure and you could blow a hole in the skin, a light cherry red is all that's needed and work the torch over a broad area and from memory approx 10 psi was all that was needed. All going well, the dings will slowly rise.
It's worth a shot if you're at the end of the road. Any remaining minor scratches will have to be removed by the metal polisher before getting them chromed, although deeper scratches may need to be filled, try a panel beater that knows how to lead fill. This is a skill the old timers were taught during their apprentiship training days. Idon't know if this method of filling is suitable under chrom plating, ask the chrome platers.
Another method is to have the top and bottom seams ground off so you end up with two half shells and panel beat them out then have them rewelded. This method works ok as long as not too much of the seam is removed, if they are you will loose the original look.
Hope this helps.
