Recommend Halfler XL-280 Repair/Restore/Rebuild ?

I had a Hafler pre and amp way back in ‘67 (whoops, that was Steely Dan talking) way back in the 80’s, great stuff, but I agree with others, unless you want it for nostalgic reasons, buy something newer in age. 

Agreed.

That is, would the $1000 necessary to create a "new" amp in the old chassis be better spent on a newly manufactured amp?

Yes. 

Okay, it sounds like the consensus is that repairing or restoring the Hafler to original specs isn’t worth the fee and freight, if I can’t do it myself.

But what about the mod shops that utilize the amp chassis, namely those from Audio By Van Alstine and Musical Concepts? That is, would the $1000 necessary to create a "new" amp in the old chassis be better spent on a newly manufactured amp?

I was thinking the same as russ69. I'm familiar with this amp from years ago, and I can understand sentimental attachment, but this amp is nothing special and it looks like you could sink a lot of money into it and end up with an expensive amp that is average or less. Think about it.

30 years old, $600 mid-fi amp? I don't think I would spend any money on it. Doesn't make sense to me. I have/had some old pieces of gear and at some point, you just have to let it go. 

     Rockford Fosgate, in Tempe, Arizona made the last iterations of the XL 280.

     You might try contacting them, for info on who still does repairs.

                            https://rockfordfosgate.com/contact-us/

musical concepts.