There's really only two options:
Add more metal and recontour
Remove more metal to recontour

Personally, I'd take a small file and just blend in the damaged portion. It wouldn't be glaringly obvious since the rear sight breaks up the top of the slide anyway.

The alternative (welding on more metal) comes with a lot of risk and expense. You could purchase a slide like this for $300 to $400. I'm not sure the end result of welding and filing would be worth it.
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