Expansions Chamber Repair and Enhancement

I have an old bike, an '83 KDX 250. While it runs well, the pipe has seen better days. I was wondering if any of you have any suggestions on repairing dents and cracks, and more importantly, if it is possible to change the shape of the pipe to get more top end. I would get an aftermarket pipe if I could find one, but the bike seems to be a rare one. Would an '83 KDX 200 pipe fit? I think I've seen aftermarket pipes for it. I can get a mig welder and sheet metal, so that's not a problem. Thanks in advance for any help.

Thanks for the reply. I'll have to look into those.