[Greg.]

Bob is right. Pressing will not work. With 1.5mm mild steel, it is unlikely any repair effort will be completely satisfactory.

A newly made aluminium copy would be good but I expect it would be cost prohibitive for a one off order.

What about retaining the existing chassis but cut out the damaged top plate, leaving a lip around the edges to allow fixing of a new cut and painted top plate? Depending on capability, this could be a DIY project or, it could be relatively affordable to contract a fabricator to do it for you. Well worth considering IMHO. Personally I could not hack living with a functioning amp that has a wonky visual appearance. I’d need to correct that.