I don't know about getting it lower, but a machine shop, like a custom motorcycle shop, may be able to help. I'm pretty sure they'll ask you to take the seat out and rail off (or you can snag one off ebay so you can drive her while you wait). I am fairly sure its a riveted assembly? They may summarize the rivets need drilled and reset, ...or if its even possible? A good shop will have a 20 ton press to repress new rivets.

You should be able to locally source another assembly here.... Car-Part.com--Used Auto Parts Market ...and run down a pick it up. But I'd find a shop first willing to do the work.